drm/i915/display: Add comment to a function that probably can be removed

This function is only called from port sync and it is identical to
what will be executed again in intel_update_crtc() over port sync
pipes.
If it is really necessary at least it deserves a better name and a
comment, leaving it to people working on port sync.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191223010654.67037-7-jose.souza@intel.com