drm/i915: Add intel implementation of the pageflip ioctl

Acked-by: Jakob Bornecrantz <jakob@vmware.com>
Acked-by: Thomas Hellström <thomas@shipmail.org>
Review-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jesse "Orange Smoothie" Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Kristian Høgsberg <krh@bitplanet.net>
Signed-off-by: Eric Anholt <eric@anholt.net>