drm/nouveau/kms: Avoid a hang entering KDB with VT accel on.

Francisco Jerez advises that pre-nv20 cards would hang if we entered
kdb with accel on and IRQs disabled, so we now disable accel before
entering kdb and re-enable it on the way back out.

Reported-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>