drm/amdgpu/gmc9: add vega12 support (v2)

Same as vega10.

v2: squash in golden regs fix from Feifei

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