Isn't complete Xv support (inc XvMC, VSync, overlays) still enabled when glamor is? I don't think replacing those with an inferior mode is sensible, but adding it as an additional adaptor is acceptable.

So fwiw I've enabled the glamor Xv backend:

commit 3e784832a52686cd29d62bdeac7b1c539c640c5b
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Sat Feb 1 16:55:32 2014 +0000

    glamor: Enable Xv support
    
    In addition to the UXA texture paths (which in theory support VSync) and
    direct overlay support, create a textured video paths that utilize the
    glamor acceleration code.
    
    Requested-by: Fabio Pedretti <fabio.ped@libero.it>
    Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=74311
    Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

[ickle@huronriver xf86-video-intel]$ xvinfo -display :0
X-Video Extension version 2.2
screen #0
  Adaptor #0: "Intel(R) Textured Video"
    number of ports: 16
    port base: 79
    operations supported: PutImage 
    supported visuals:
      depth 24, visualID 0x21
    no port attributes defined
    maximum XvImage size: 2048 x 2048
    Number of image formats: 4
      id: 0x32595559 (YUY2)
        guid: 59555932-0000-0010-8000-00aa00389b71
        bits per pixel: 16
        number of planes: 1
        type: YUV (packed)
      id: 0x32315659 (YV12)
        guid: 59563132-0000-0010-8000-00aa00389b71
        bits per pixel: 12
        number of planes: 3
        type: YUV (planar)
      id: 0x30323449 (I420)
        guid: 49343230-0000-0010-8000-00aa00389b71
        bits per pixel: 12
        number of planes: 3
        type: YUV (planar)
      id: 0x59565955 (UYVY)
        guid: 55595659-0000-0010-8000-00aa00389b71
        bits per pixel: 16
        number of planes: 1
        type: YUV (packed)
  Adaptor #1: "GLAMOR Textured Video"
    number of ports: 16
    port base: 95
    operations supported: PutImage 
    supported visuals:
      depth 24, visualID 0x21
    number of attributes: 5
      "XV_BRIGHTNESS" (range -1000 to 1000)
              client settable attribute
              client gettable attribute (current value is 0)
      "XV_CONTRAST" (range -1000 to 1000)
              client settable attribute
              client gettable attribute (current value is 0)
      "XV_SATURATION" (range -1000 to 1000)
              client settable attribute
              client gettable attribute (current value is 0)
      "XV_HUE" (range -1000 to 1000)
              client settable attribute
              client gettable attribute (current value is 0)
      "XV_COLORSPACE" (range 0 to 1)
              client settable attribute
              client gettable attribute (current value is 0)
    maximum XvImage size: 8192 x 8192
    Number of image formats: 2
      id: 0x32315659 (YV12)
        guid: 59563132-0000-0010-8000-00aa00389b71
        bits per pixel: 12
        number of planes: 3
        type: YUV (planar)
      id: 0x30323449 (I420)
        guid: 49343230-0000-0010-8000-00aa00389b71
        bits per pixel: 12
        number of planes: 3
        type: YUV (planar)