drm/i915: move backlight to display.backlight

Move display backlight related members under drm_i915_private display
sub-struct.

Prefer adding anonymous sub-structs even for single members that aren't
our own structs.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/026241565dad12e0024c443419fa5e0caac41b2d.1661779055.git.jani.nikula@intel.com