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: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170720175754.30751-7-daniel.vetter@ffwll.ch