drm/amdgpu: Add check for USWC support for amdgpu_display_supported_domains

This verifies we don't add GTT as allowed domain for APUs when USWC
is disabled.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Acked-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>