drm/nouveau/kms/nv50-: Check vbl count after CRC context flip

While I haven't seen us take too long in nv50_crc_ctx_flip_work() outside
of users with kernel logging on a serial port, it probably would be a good
idea to check how long we take just in case we need to go faster in the
future.

Cc: Martin Peres <martin.peres@free.fr>
Cc: Jeremy Cline <jcline@redhat.com>
Cc: Simon Ser <contact@emersion.fr>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://gitlab.freedesktop.org/drm/nouveau/-/merge_requests/10