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>
12 jobs for fix-dri2-scanout-pixmap in 1 minute and 2 seconds (queued for 8 seconds)
Status Name Job ID Coverage
  Docker Image
passed container-build #13158824

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  Build
passed xserver-1.13 #13158827

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passed xserver-1.14 #13158828

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passed xserver-1.15 #13158829

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passed xserver-1.16 #13158830

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passed xserver-1.17 #13158831

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passed xserver-1.18 #13158832

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passed xserver-1.18-clang #13158833

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passed xserver-1.19 #13158834

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passed xserver-1.19-clang #13158835

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passed xserver-1.20 #13158825

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passed xserver-1.20-clang #13158826

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