drm/i915: Add Guess-o-matic for pageflip timestamping.

This patch changes the strategy for pageflip completion
timestamping. It detects if the pageflip completion
routine gets executed before or after drm_handle_vblank,
and thereby decides if the returned vblank count and
timestamp must be incremented by 1 frame(duration) or
not. It compares the current system time at invocation
against the current vblank timestamp. If the difference
is more than 0.9 video refresh interval durations then
it assumes the vblank timestamp and count are outdated
and need to be incremented and does so. Otherwise it
assumes a delayed pageflip irq and doesn't correct
the timestamp and count.

Advantage of this patch: Pageflip timestamping becomes
more robust against implementation errors and is
maintenance free for future GPU's.

Disadvantage: A few dozen (hundred?) nsecs extra
time spent in pageflip irq handler for each flip,
compared to hard-coded per-gpu settings?

Signed-off-by: Mario Kleiner <mario.kleiner@tuebingen.mpg.de>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>