drm/i915/ddi: abstract figuring out encoder name

The encoder name deduction has become a bit cumbersome within
intel_ddi_init(). Split it out to a separate function to declutter
intel_ddi_init(), even if that means having to use a temp seq buffer for
the name.

Cc: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Reviewed-by: Ankit Nautiyal <ankit.k.nautiyal@intel.com>
Link: https://lore.kernel.org/r/20250903101050.3671305-1-jani.nikula@intel.com


Signed-off-by: Jani Nikula <jani.nikula@intel.com>