glamor: Set AMDGPU_CREATE_PIXMAP_SCANOUT on DRI2 and shared pixmaps

The scanout usage flag wasn't applied for DRI2 and shared pixmaps
in 0732f81a, resulting in GPU artifacts in some scenarios.

Fixes: 0732f81a ("glamor: Make pixmap scanout compatible if its
                      dimensions are")
Closes: xorg/driver/xf86-video-amdgpu#41
Signed-off-by: Dor Askayo <dor.askayo@gmail.com>
Signed-off-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>