drm/i915: Clear the vblank status bit before polling for the next vblank

The vblank status bit is a sticky bit that must be cleared with a write
of '1' prior to polling for the next vblank.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Sitsofe Wheeler <sitsofe@yahoo.com>
jbarnes: I'd still rather see a lock, but I think you're right that
we don't generally wait in code that needs not to miss an interrupt.
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>