drm/i915/dsi: convert intel_dsi_dcs_backlight.c to struct intel_display

Going forward, struct intel_display is the main display device data
pointer. Convert intel_dsi_dcs_backlight.c to struct intel_display.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://lore.kernel.org/r/19ed78f51ac153016fbe60c49037bef840a9cc1b.1742554320.git.jani.nikula@intel.com


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