Simplify the hotplug code connector->encoder->hpd_pin handling
by introducing a helper for exactly this purpose.

In the helper we can neatly deal with the potential lack of an
attached encoder on fresh MST connectors leaving the rest of the
hpd code oblivious to such details.

Cc: Lyude Paul <lyude@redhat.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200121171100.4370-3-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>