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

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>
cc: stable@kernel.org
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>