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

drivers/gpu/drm/i915/intel_panel.c

@@ -278,6 +278,6 @@ void intel_panel_setup_backlight(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
 
-	dev_priv->backlight_level = intel_panel_get_max_backlight(dev);
+	dev_priv->backlight_level = intel_panel_get_backlight(dev);
 	dev_priv->backlight_enabled = dev_priv->backlight_level != 0;
 }