drm/nouveau: Add module options to disable acceleration.

noaccel=1 disables all acceleration and doesn't even attempt
initialising PGRAPH+PFIFO, nofbaccel=1 only makes fbcon unaccelerated.

Signed-off-by: Marcin Kościelnicki <koriakin@0x04.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>