drm/amdgpu: add check before free wb entry

Check if ring is not a mes queue before freeing the wb entry,
because we only allocate a wb entry when it's not a mes queue.

Signed-off-by: Jesse Zhang <jesse.zhang@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>