drm/i915: Remove cs based page flip support.

With mmio flips now available on all platforms it's time to remove
support for cs flips.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463490484-19540-13-git-send-email-maarten.lankhorst@linux.intel.com


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