drm/amdgpu/soc15: disable doorbell interrupt as part of BACO entry sequence

Workaround to make RAS recovery work in BACO reset.

Signed-off-by: Le Ma <le.ma@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>