drm/nouveau: make vbios parser runnable from an atomic context

The nv50 display isr bh needs to be converted to a tasklet, which means
we can't sleep anymore.  The places we execute vbios init tables are
rare, and not in any way performance critical, so this isn't a huge
problem.

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