glamor: Make pixmap scanout compatible if its dimensions are

Namely, if its dimensions match those of the screen pixmap (enough that
it could stand in for it). When that's the case, the pixmap may end up
being scanned out directly due to page flipping via the Present
extension, e.g. with xfwm4 --vblank=xpresent .

v2:
* Use AMDGPU_CREATE_PIXMAP_SCANOUT instead of second-guessing in
  amdgpu_alloc_pixmap_bo, fixes corruption when resizing from smaller
  to larger virtual size via RandR.

Closes: xorg/driver/xf86-video-amdgpu#10