drm/i915/panel: Only record the backlight level when it is enabled

drm/i915/panel: Only record the backlight level when it is enabled

commit 47356eb67285014527a5ab87543ba1fae3d1e10a upstream.

By tracking the current status of the backlight we can prevent recording
the value of the current backlight when we have disabled it. And so
prevent restoring it to 'off' after an unbalanced sequence of
intel_lvds_disable/enable.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=22672
Tested-by: Alex Riesen <raa.lkml@gmail.com>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>