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
12 jobs for glamor-pixmap-scanout in 1 minute and 20 seconds (queued for 1 second)
Status Name Job ID Coverage
  Docker Image
passed debian-testing #2374091

00:00:21

 
  Build
passed xserver-1.13 #2374094

00:00:46

passed xserver-1.14 #2374095

00:00:49

passed xserver-1.15 #2374096

00:00:31

passed xserver-1.16 #2374097

00:00:47

passed xserver-1.17 #2374098

00:00:32

passed xserver-1.18 #2374099

00:00:46

passed xserver-1.18-clang #2374100

00:00:51

passed xserver-1.19 #2374101

00:00:49

passed xserver-1.19-clang #2374102

00:00:52

passed xserver-1.20 #2374092

00:00:58

passed xserver-1.20-clang #2374093

00:00:50