Check for AMDGPU_CREATE_PIXMAP_SCANOUT in amdgpu_glamor_create_pixmap

We must not call glamor_create_pixmap or fbCreatePixmap when it's set.

Closes: xorg/driver/xf86-video-amdgpu#21
12 jobs for amdgpu_glamor_create_pixmap-AMDGPU_CREATE_PIXMAP_SCANOUT in 1 minute and 19 seconds (queued for 2 seconds)