drm/amdgpu/vcn2.6: Add vcn2.6 support

Aldebaran is using vcn2.6, and the main change is vcn2.6 using
AMDGPU_MMHUB_0, and vcn2.5 using AMDGPU_MMHUB_1

Signed-off-by: James Zhu <James.Zhu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>