drm/i915/backlight: extract backlight code to a separate file

In a long overdue refactoring, split out backlight code to new
intel_backlight.[ch]. Simple code movement, leave renames for follow-up
work. No functional changes.

Cc: Lyude Paul <lyude@redhat.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/97d310848f03061473b9b2328e2c5c4dcf263cfa.1629888677.git.jani.nikula@intel.com