drm/radeon/kms: Don't try to process irq when we are unloading

If module is being unloaded we should not try to handle irq especialy
we should not call into drm helper or we could hard hang the computer
free_irq will call the irq handler to make sure we behave properly.

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