drm/i915: Use atomic commits for legacy page_flips

Note that I didn't start garbage collecting all the legacy flip code
yet, to make it easier to revert this. But there will be _lots_ of
code that can be removed once this is tested on all platforms.

v2: Use __maybe_unused (Maarten).

Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1465827229-1704-5-git-send-email-daniel.vetter@ffwll.ch