drm/amd/amdgpu: add Polaris12 support (v3)

v2: agd: squash in various fixes
v3: agd: squash in:
drm/amdgpu: remove unnecessary smc sk firmware for polaris12

Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Ken Wang <Qingqing.Wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>