drm/amdgpu/virt: don't check VALID bit for FLR completion message

The interrupt after FLR is missed sometimes due to hardware reason, so
guest driver get the notification of FLR completion via polling
message. Then host doesn't write VALID bit to avoid sending interrupt,
otherwise the completion will be handled twice.

So there's a valid message without VALID bit for FLR completion,
driver should handle it without checking.

Signed-off-by: Pixel Ding <Pixel.Ding@amd.com>
Reviewed-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>