drm/i915: Skip .get_backlight() when backlight isn't enabled

On VLV/CHV when the display is off, we can't read out the current
backlight level from the hardware since we have no pipe to do so.
Currently we end up reading a bigus register due to passing
INVALID_PIPE to VLV_BLC_PWM_CTL().

Skip the entire .get_backlight() call if the backlight isn't enabled
according to backlight.enabled.

This problem can be reproduced simply by reading the backlight device
actual_brightness file while the display is off.

Cc: Jani Nikula <jani.nikula@intel.com>
Suggested-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: NJani Nikula <jani.nikula@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_panel.c

@@ -539,12 +539,15 @@ static u32 intel_panel_get_backlight(struct intel_connector *connector)
 {
 	struct drm_device *dev = connector->base.dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
-	u32 val;
+	struct intel_panel *panel = &connector->panel;
+	u32 val = 0;
 
 	mutex_lock(&dev_priv->backlight_lock);
 
-	val = dev_priv->display.get_backlight(connector);
-	val = intel_panel_compute_brightness(connector, val);
+	if (panel->backlight.enabled) {
+		val = dev_priv->display.get_backlight(connector);
+		val = intel_panel_compute_brightness(connector, val);
+	}
 
 	mutex_unlock(&dev_priv->backlight_lock);