drm/nouveau/timer: restore the time on resume

This can be useful if some parts of Nouveau try to calculate the time
between two events.  Without this patch, the time difference would be
negative in the case where the computer is suspended/resumed between
two events.

This patch should fix fan speed probing when done while suspending/resuming.

Solve this by saving the current time before suspending and by restoring it
on resume.

Signed-off-by: Martin Peres <martin.peres@labri.fr>
Tested-by: Martin Peres <martin.peres@labri.fr>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>