drm/i915: fix up _wait_for macro

drm/i915: fix up _wait_for macro

As Thomas Gleixner spotted, it's rather horrible racy:
- We can miss almost a full tick, so need to compensate by 1 jiffy.
- We need to re-check the condition when having timed-out, since a
  the last check could have been before the timeout expired. E.g. when
  we've been preempted or a long irq happened.

Cc: Thomas Gleixner <tglx@linutronix.de>
Reported-by: Jack Winter <jbh@alchemy.lu>
Cc: Jack Winter <jbh@alchemy.lu>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>