drm/amdgpu: Disable unmapped doorbell handling basic mode on mes 12

The new mechanism for unmapped doorbell handling requires both driver side and
MES fw side change. The FW side changes are still not released.

Signed-off-by: shaoyunl <shaoyun.liu@amd.com>
Reviewed-by: Harish Kasiviswanthan <Harish.Kasiviswanthan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>