drm/radeon/kms: free ib pool on module unloading

commit ccd6895d upstream.

ib pool weren't free for various newer asic on module unload.
This doesn't cause much arm but still could be candidate for
stable.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>