drm/i915/overlay: Switch to intel_frontbuffer_flip()

drm/i915/overlay: Switch to intel_frontbuffer_flip()

Get rid of intel_frontbuffer_flip_{prepare,complete}() from
the overlay code and just use intel_frontbuffer_flip() instead.

The only difference between these are the light interactions
with the ORIGIN_CS busyness tracking, but since the only user
of this is the overlay/xf86-video-intel/Xv the buffer will
always be filled by the CPU and thus we'll never see any
ORIGIN_CS frontbuffer activity there anyway. Also I don't
think we actually have anything covered by the frontbuffer
tracking that affects the overlay (FBC is on the primary
plane, DRRS isn't currently enabled on the platforms with
overlay, and PSR doesn't exist in the hardware).

Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251016185408.22735-3-ville.syrjala@linux.intel.com