drm/nouveau: disallow fbcon accel if running in interrupt context

A future commit will add locking to the DRM's channel, and there's numerous
problems that come up if we allow printk from an interrupt context to be
accelerated.  It seems saner to just disallow it completely.

As a nice side-effect, all the "to accel or not to accel" logic gets moved
out of the chipset-specific code.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>