drm/amdgpu: set cpg doorbell for fiji and polaris.

add set_doorbell functions for mec and cpg.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>