drm/amdkfd: make sure ring buffer is flushed before update wptr

In a consecutive packet submission, for example unmap and query status,
when CP is reading wptr caused by unmap packet doorbell ring, if in some
case CP operates slower (e.g. doorbell_mode=1) and wptr has been updated
to next packet (query status), but the query status packet content has
not been flushed to memory yet, it will cause CP fetched stalled data.

Adding mb to ensure ring buffer has been updated before updating wptr.
Also adding a mb to ensure wptr updated before doorbell ring.

Signed-off-by: Victor Zhao <Victor.Zhao@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>