drm/i915/display: convert dp aux backlight to struct intel_display

Going forward, struct intel_display shall replace struct
drm_i915_private as the main display device data pointer type. Convert
intel_dp_aux_backlight.[ch] to struct intel_display.
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240813164123.2674462-6-jani.nikula@intel.comSigned-off-by: Jani Nikula <jani.nikula@intel.com>