drm/i915/overlay: Track current frontbuffer_bits

drm/i915/overlay: Track current frontbuffer_bits

Store the current frontbuffer_bits in the overlay data. The
main benefit here is that we get rid of the 'crtc->pipe'
usage from intel_overlay_flip_prepare() which will have to
move to the i915 side of the parent vs. display driver split.

And since the goal is to get rid of the crtc stuff, move those
out from intel_overlay_off_tail() into intel_overlay_switch_off()
since the i915 side doesn't use those anymore, and the display
side doesn't need those anymore after that anyway.
intel_overlay_off_tail() will itself move to the i915 side of
the fence once the driver split is done.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260226100738.29997-3-ville.syrjala@linux.intel.com