R9285 resumes OK after doing echo mem >/sys/power/state, but will not sleep/resume  a second time.

Tested with a few kernels 4.4 wip 4.3 and 4.2.

Nothing is logged in dmesg for 4.4 and 4.3 X bails after some time with -

[ 29371.315] (EE) Backtrace:
[ 29371.326] (EE) 0: /usr/bin/X (xorg_backtrace+0x49) [0x584559]
[ 29371.326] (EE) 1: /usr/bin/X (0x400000+0x188459) [0x588459]
[ 29371.326] (EE) 2: /lib/libpthread.so.0 (0x7f74cfb8a000+0xf2a0) [0x7f74cfb992a0]
[ 29371.326] (EE) 3: /lib/libc.so.6 (0x7f74ce1cb000+0x8a7e7) [0x7f74ce2557e7]
[ 29371.327] (EE) 4: /usr/lib/xorg/modules/libglamoregl.so (0x7f74cdd80000+0x196ab) [0x7f74cdd996ab]
[ 29371.327] (EE) 5: /usr/bin/X (0x400000+0x111a9f) [0x511a9f]
[ 29371.327] (EE) 6: /usr/bin/X (miPaintWindow+0x20c) [0x56802c]
[ 29371.327] (EE) 7: /usr/bin/X (miHandleValidateExposures+0x47) [0x57c767]
[ 29371.327] (EE) 8: /usr/bin/X (SetRootClip+0x2b5) [0x465905]
[ 29371.327] (EE) 9: /usr/bin/X (0x400000+0xb81d2) [0x4b81d2]
[ 29371.327] (EE) 10: /usr/bin/X (xf86VTEnter+0x12b) [0x47420b]
[ 29371.327] (EE) 11: /usr/bin/X (WakeupHandler+0x6b) [0x43b59b]
[ 29371.327] (EE) 12: /usr/bin/X (WaitForSomething+0x1ba) [0x58173a]
[ 29371.327] (EE) 13: /usr/bin/X (0x400000+0x3699e) [0x43699e]
[ 29371.328] (EE) 14: /usr/bin/X (0x400000+0x3aacb) [0x43aacb]
[ 29371.328] (EE) 15: /lib/libc.so.6 (__libc_start_main+0xed) [0x7f74ce1ec36d]
[ 29371.328] (EE) 16: /usr/bin/X (0x400000+0x262a1) [0x4262a1]
[ 29371.328] (EE) 
[ 29371.328] (EE) Bus error at address 0x7f74c7003320
[ 29371.328] (EE) 
Fatal server error:
[ 29371.328] (EE) Caught signal 7 (Bus error). Server aborting


fbcon is still functional at this point but trying again to startx will hang, last lines of xorg log =

[ 10618.330] (II) Loading sub module "dri2"
[ 10618.330] (II) LoadModule: "dri2"
[ 10618.330] (II) Module "dri2" already built-in

On 4.2 I see logged at this point lots of -

GPU lockup (current fence id 0x0000000000000147 last fence id 0x0000000000000148 on ring 0)
ring 0 stalled for more than 61125msec

Waiting with 4.3 or 4.4 does not produce a hung task backtrace.