drm/amdkfd: destroy mqd when destroying kernel queue

This patch adds a missing destruction of mqd, when destroying a kernel queue.
Without the destruction, there is a memory leakage when repeatedly creating and
destroying kernel queues.

Signed-off-by: Ben Goz <ben.goz@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org