drm/amdgpu: Clear doorbell interrupt status for Sienna Cichlid

On Sienna Cichlid, in pass-through mode, if we unload the driver in BACO
mode(RTPM), then the kernel would receive thousands of interrupts.
That's because there is doorbell monitor interrupt on BIF, so KVM keeps
injecting interrupts to the guest VM. So we should clear the doorbell
interrupt status after BACO exit.

v2: Modify coding style and commit message

Signed-off-by: Chengzhe Liu <ChengZhe.Liu@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>