drm/i915: Consult the registered encoders for the ICL combo PHY w/a

drm/i915: Consult the registered encoders for the ICL combo PHY w/a

Display WA #1178 calls us to tweak some magic bits when doing AUX
to an external combo PHY port. Instead of looking to see if the VBT
has declared such a port (which could in theory even alias with a
declared eDP port on the same PHY) just check the real situation
based on the registered encoders.

The only slight chicken vs. egg situation here is during output
probing. But typically we'd register the eDP ports first and so
once we get to probe anything external on the combo PHY we have
already determined if it's eDP or not.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230208015508.24824-7-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>