drm/i915: Remove intel_flip_work infrastructure

This gets rid of all the interactions between the legacy flip code and
the modeset code. Yay!

This highlights an ommission in the atomic paths, where we fail to
apply a boost to the pending rendering when we miss the target vblank.
But the existing code is still dead and can be removed.

v2: Note that the boosting doesn't work in atomic (Chris).

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/20170720175754.30751-7-daniel.vetter@ffwll.ch