drm/i915/panel: The backlight is enabled if the current value is non-zero

... and not if the maximum is non-zero. This fixes the typo introduced
in 47356eb6728501452 and preserves the backlight value from boot.

[ickle: My thanks also to Indan Zupancic for diagnosing the original
        regression and suggesting the appropriate fix.]
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org # after 47356eb6728501452