drm/amdgpu: Fix hard hang for S/G display BOs.

HW requires for caching to be unset for scanout BO
mappings when the BO placement is in GTT memory.
Usually the flag to unset is passed from user mode
but for FB mode this was missing.

v2:
Keep all BO placement logic in amdgpu_display_supported_domains

Suggested-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Shirish S <shirish.s@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>