bd->props.brightness doesn't reflect the actual backlight level.

Always invoke backlight_update_status when users want to change
the backlight.

For setups where brightness change is an expensive operation,
this could be done in the driver rather than the core.

http://bugzilla.kernel.org/show_bug.cgi?id=12249

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Signed-off-by: Richard Purdie <rpurdie@linux.intel.com>