nvidiafb: prevent triggering of softlockup

If the chip locks up, we get into a long polling loop,
where the softlockup detector kicks in.
See https://bugzilla.redhat.com/bugzilla/attachment.cgi?id=151878


for an example.

[adaplas]
Chip lockup can occur at 3 points (flush, sync, and wait). Consolidate and
allow the driver to go to safe mode cleanly.

Signed-off-by: Dave Jones <davej@redhat.com>
Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>