drm/nouveau: set irq_enabled manually

Since commit 0fa9061a ("drm/nouveau/mc: handle irq-related setup
ourselves"), drm_device->irq_enabled remained unset. This is needed in
order to properly wait for a vblank event in the generic drm code.

See https://bugs.freedesktop.org/show_bug.cgi?id=74195



Reported-by: Jan Janecek <janjanjanx@gmail.com>
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Cc: stable@vger.kernel.org # 3.10+
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>