drm/amdgpu: Use GTT for dumb buffer if sg display enabled (v2)

When vram size <= THRESHOLD(256M) lets use GTT for dumb buffer
allocation. As SG will be enabled with vram size <= 256M
scan out will not be an issue.

v2: Use amdgpu_display_supported_domains to get supported domain.

Signed-off-by: Deepak Sharma <Deepak.Sharma@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>