drm/radeon/kms: Don't try to enable IRQ if we have no handler installed

If for any reason we haven't installed handler we shouldn't try to
enable IRQ/MSI on the hw so we don't get unhandled IRQ/MSI which
makes the kernel sad.

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