drm/i915: return actual brightness to .get_brightness callback

The backlight device .get_brightness callback is supposed to return the
actual brightness by querying the hardware, not the cached value. See
Documentation/ABI/stable/sysfs-class-backlight. The callback is there to
support the actual_brightness sysfs file.

With the backlight_level and backlight device brightness now in sync, one
can reliably get the cached value through the brightness sysfs file.

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