drm/i915/fbc: Start using flip nuke

drm/i915/fbc: Start using flip nuke

The hardware automagically nukes the cfb on flip. We can use
that whenever the plane/crtc configuration doesn't change too
much. Let's hook that up.

We'll need this for glk+ since we need to introduce an extra
vblank wait after FBC disable. As we're currently disabling
FBC around all plane updates we'd slow them down by an extra
frame. Not a great user experience when your fps is always
capped at vrefres/2. With flip nuke we don't need the extra
vblank wait.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191127201222.16669-12-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>