Solves the xserver/hw/xfree86/modes scrn->monitor and scrn->modes
inconsistency.  

The commit 37d956e3ac9513b74078882dff489f9b0a7a5a28 into xf86Crtc.c 
xf86CrtcConfigInit() modified the initialization of 
config->compat_output from default 0 to -1.  

This change exposes a bug.  During initial configuration a monitor 
detection is attempted and if an EDID block is available the driver 
calls xf86OutputSetEDID().  There the output in process is checked for 
equality to the compat_output, and if so then the output EDID is 
copied into the screen monitor properties, through 
xf86SetDDCproperties() and xf86EdidMonitorSet().  This saved screen 
monitor EDID can be queried via

  xprop -root XFree86_DDC_EDID1_RAWDATA

Tracing the call tree:

xf86InitialConfiguration(ScrnInfoPtr scrn, Bool canGrow)
-> xf86ProbeOutputModes(scrn, width, height);
 --> output_modes = (*output->funcs->get_modes) (output);
  ---> xf86OutputSetEDID(xf86OutputPtr output, xf86MonPtr edid_mon)
   ----> if (output == xf86CompatOutput(scrn))
   ---->     xf86SetDDCproperties(scrn, edid_mon);   XX for screen monitor
-> xf86SetScrnInfoModes(scrn);
 --> output = SetCompatOutput(config);               XX sets compat_output
  ---> config->compat_output = compat;

Hence during initial boot xf86SetDDCproperties() used to be called for 
output 0 but is now never called since there is no output -1.  

To reproduce: 
1. in the Section "Device" add the Option "ModeDebug" "on",
2. connect a single EDID monitor,
3. ~# X :1 -retro
4. un- and replug the monitor.

Xorg.1.log:

[347954.434] (II) RADEON(0): Output VGA-0 has no monitor section
[347954.434] (II) RADEON(0): xf86InitialConfiguration config->compat_output=-1
[347954.434] (**) RADEON(0): Option "ModeDebug" "on"
[347954.466] (II) RADEON(0): EDID for output DVI-0
...
[347954.467] (II) RADEON(0): 	00363230303036313959420a2020003f
[347954.467] (II) RADEON(0): Not using mode "720x576@25i" (hsync out of range)
...

XX disconnect
[347977.171] (II) RADEON(0): EDID for output DVI-0
[347977.173] (II) RADEON(0): EDID for output HDMI-0
...

XX reconnect
[347992.820] (II) RADEON(0): EDID for output DVI-0
...
[347992.821] (II) RADEON(0): 	00363230303036313959420a2020003f
XX xf86SetDDCproperties() -> xf86EdidMonitorSet() -> xf86DDCGetModes()
[347992.821] (II) RADEON(0): EDID vendor "NEC", prod id 26288
[347992.821] (II) RADEON(0): Using hsync ranges from config file
[347992.821] (II) RADEON(0): Using vrefresh ranges from config file
[347992.821] (II) RADEON(0): Printing DDC gathered Modelines:
...
[347992.821] (II) RADEON(0): Not using mode "720x576@25i" (hsync out of range)
...

So consistency between scrn->monitor and scrn->modes requires a call 
sequence xf86SetDDCproperties() followed by xf86SetScrnInfoModes().  
Two places seem fit: at the begin of xf86SetScrnInfoModes() or at the 
end of SetCompatOutput().

This patch takes the latter route.  It combines the if-then-else tree 
for readability, but execution is unchanged since

1. output == NULL  ==>  compat == -1
2. num_output > 0  ==>  config->output[0 .. num_output-1] != NULL

Also, the compat_output preset in xf86TargetFallback() is removed 
since its selection differs from the SetCompatOutput() logic.