drm/i915: Eliminate lots of WARNs when there's no backlight present

My 855gm doesn't register the intel backlight but it still ends up
calling the backlight code to enable/disable the backlight via the
LVDS code. This leads to some WARNs due to backlight.max being 0.

Let's have intel_panel_enable_backlight() and intel_panel_disable_backlight()
check whether there's a backlight present or not.

Also move the backlight.present check from asle_set_backlight() into
intel_panel_set_backlight() for some extra symmetry.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_opregion.c

@@ -396,9 +396,7 @@ int intel_opregion_notify_adapter(struct drm_device *dev, pci_power_t state)
 static u32 asle_set_backlight(struct drm_device *dev, u32 bclp)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
-	struct drm_connector *connector;
 	struct intel_connector *intel_connector;
-	struct intel_panel *panel;
 	struct opregion_asle __iomem *asle = dev_priv->opregion.asle;
 
 	DRM_DEBUG_DRIVER("bclp = 0x%08x\n", bclp);
@@ -417,12 +415,8 @@ static u32 asle_set_backlight(struct drm_device *dev, u32 bclp)
 	 * only one).
 	 */
 	DRM_DEBUG_KMS("updating opregion backlight %d/255\n", bclp);
-	list_for_each_entry(connector, &dev->mode_config.connector_list, head) {
-		intel_connector = to_intel_connector(connector);
-		panel = &intel_connector->panel;
-		if (panel->backlight.present)
-			intel_panel_set_backlight(intel_connector, bclp, 255);
-	}
+	list_for_each_entry(intel_connector, &dev->mode_config.connector_list, base.head)
+		intel_panel_set_backlight(intel_connector, bclp, 255);
 	iowrite32(DIV_ROUND_UP(bclp * 100, 255) | ASLE_CBLV_VALID, &asle->cblv);
 
 	mutex_unlock(&dev->mode_config.mutex);