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

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>
Cc: stable@kernel.org