drm/atomic-helper: Set fence deadline for vblank

For an atomic commit updating a single CRTC (ie. a pageflip) calculate
the next vblank time, and inform the fence(s) of that deadline.

v2: Comment typo fix (danvet)
v3: If there are multiple CRTCs, consider the time of the soonest vblank

Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>