drm/i915: keep backlight_level and backlight device brightness in sync

A single point of truth would be better than two, but achieving that would
require more abstractions for CONFIG_BACKLIGHT_CLASS_DEVICE=n with not a
whole lot of real benefits. Take the short route and just keep the
backlight levels in sync. In particular, update backlight device brightness
on opregion brightness changes.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>