drm/i915/backlight: convert to use struct intel_display

Going forward, struct intel_display will be the main display data
structure. Convert as much as possible of backlight code to use it.

There are some stragglers mainly around PCH checks.

Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250204132423.2910978-1-jani.nikula@intel.com


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