drm/i915/ddi: stop using dp_to_i915()

Switch to struct intel_display and to_intel_display() instead of using
dp_to_i915().

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/6557281bc3f8df88931c045deb08cf76b727cda2.1725012870.git.jani.nikula@intel.com