drm/amdgpu: Add helper function to get buffer domain

Move logic of getting supported domain to a helper
function

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>