drm/{i915, xe}: move xe_display_flush_cleanup_work() to i915 display

drm/{i915, xe}: move xe_display_flush_cleanup_work() to i915 display

xe_display_flush_cleanup_work() is a bit of an oddball function in xe
display code. There shouldn't be anything this specific or xe
specific. While I'm not sure what the correct refactor for the function
should be, move it to shared display code for starters, next to the
eerily similar but slightly different intel_has_pending_fb_unpin() that
is only called from i915 core.

The main goal here is to unblock some refactors on
for_each_intel_crtc().

v2: Add FIXME comment (Ville)

Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20260525110553.651208-1-jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>