drm/amdgpu: add register definition for VCN RAS initialization

Prepare for enableing VCN RAS poison.

v2: move SHIFT and MASK definitions to related sh_mask.h file.

Signed-off-by: Tao Zhou <tao.zhou1@amd.com>
Signed-off-by: Lijo Lazar <lijo.lazar@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>