drm/vblank: Add helper to get next vblank time

Will be used in the next commit to set a deadline on fences that an
atomic update is waiting on.

v2: Calculate time at *start* of vblank period, not end
v3: Fix kbuild complaints

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>