drm/radeon/kms: Fix irq handling on AVIVO hw

Avivo hw have vblank interrupt in different place, fixes
irq handling (especialy irq disabling while suspending or
shuting down the module).

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