drm/amdkfd: Asynchronously free smi_client

The synchronize_rcu may take several ms, which noticeably slows down
applications close SMI event handle. Use call_rcu to free client->fifo
and client asynchronously and eliminate the synchronize_rcu call in the
user thread.

Signed-off-by: Philip Yang <Philip.Yang@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>