drm/i915/hti: abstract hti handling

The HTI or HDPORT handling is sprinkled around. Centralize to one place.

Add a note about how subtle the mapping from HDPORT_STATE register to
dpll mask actually is.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221109144209.3624739-1-jani.nikula@intel.com