drm/i915: Eliminate race from gen2/3 page flip interrupt handling

If the interrupt handler were to process a previous vblank interrupt and
the following flip pending interrupt at the same time, the page flip
would be completed too soon.

To eliminate this race, check the live pending flip status from the ISR
register before finishing the page flip.

v2: Added a comment explaining the logic (by Chris Wilson)
v3: Fix a typo in the comment

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>