drm/i915/overlay: Extract i915_overlay_cleanup()

drm/i915/overlay: Extract i915_overlay_cleanup()

Pull the i915 specific bits of the overlay cleanup into
a separate function (i915_overlay_cleanup()) to accommodate
the upcoming parent vs. display driver split.

For now we'll also have to pass in the overlay struct, but
that will disappear once the i915 vs. display split is completed.

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260226100738.29997-11-ville.syrjala@linux.intel.com