drm/i915: Convert flip_work to a list.

This will be required to allow more than 1 update in the future.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463490484-19540-10-git-send-email-maarten.lankhorst@linux.intel.com


Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>