Add initial gfxhub v12 support.

v1: Add gfxhub v12_0 ip block support (Likun)
v2: Switch to AMDGPU_GFXHUB(0) (Hawking)
v3: Squash in keep default error response mode (Hawking)
Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>