drm/amdgpu/vcn:Update latest UVD_MPC register for VCN

Update latest UVD_MPC register for VCN. Use defined
macro to replace value for readability.

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