drm/amdgpu: enable S/G display on PCO and RV2 (v2)

It should work on all Raven variants, but some users have
reported issues with original Raven with IOMMU enabled.
So far there have been no issues observed with PCO or RV2.

v2: split out the dm init and domain changes into separate
    patches.

Acked-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>