drm/i915/frontbuffer: Handle the dirtyfb cache flush inside intel_frontbuffer_flush()

drm/i915/frontbuffer: Handle the dirtyfb cache flush inside intel_frontbuffer_flush()

intel_bo_frontbuffer_flush_for_display() is a bit too low level
to be directly in the high level dirtyfb code. Move the calls
into intel_frontbuffer_flush().

There is a slight behavioural change here in that we now skip
the flush if the bo is not a current scanout buffer (front->bits
== 0). But that is fine as the flush will eventually happen via
the fb pinning code if/when the bo becomes a scanout buffer again.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patch.msgid.link/20251016185408.22735-6-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>