drm/i915: Fix vbt PWM max setup for CTG

CTG uses hrawclk for backlight, so calculate the max based on that
instead of cdclk.

Fixes: aa17cdb4f836 ("drm/i915: initialize backlight max from VBT")
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448893432-6978-3-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_panel.c

@@ -1352,13 +1352,19 @@ static u32 i9xx_hz_to_pwm(struct intel_connector *connector, u32 pwm_freq_hz)
 
 /*
  * Gen4: This value represents the period of the PWM stream in display core
- * clocks multiplied by 128.
+ * clocks ([DevCTG] HRAW clocks) multiplied by 128.
+ *
  */
 static u32 i965_hz_to_pwm(struct intel_connector *connector, u32 pwm_freq_hz)
 {
 	struct drm_device *dev = connector->base.dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
-	int clock = 1000 * dev_priv->display.get_display_clock_speed(dev);
+	int clock;
+
+	if (IS_G4X(dev_priv))
+		clock = MHz(intel_hrawclk(dev));
+	else
+		clock = 1000 * dev_priv->display.get_display_clock_speed(dev);
 
 	return clock / (pwm_freq_hz * 128);
 }