drm/i915: Nuke legacy flip queueing code

Just a very minimal patch to nuke that code. Lots of the flip
interrupt handling stuff is still around.

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170719125502.25696-2-daniel.vetter@ffwll.ch