drm/amdgpu: add mmhub v2 block for navi10 (v4)

mmhub is the memory controller hub for multi-media (VCN).

v1: add place holder and initial functions (Ray)
v2: replace legacy amdgpu_mc structure with amdgpu_gmc (Hawking)
v3: switch to use amdgpu_gmc_pd_addr (Hawking)
v4: squash in updates (Alex)

Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Hawking Zhang <Hawking.Zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>