drm/radeon/kms: Make sure we release AGP device if we acquired it

In some case we weren't releasing the AGP device at module unloading.
This leaded to unfunctional AGP at next module load. This patch make
sure we release the AGP bus if we acquire it.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>