drm/i915: Use cached cdclk_freq for PWM calculations

No need to read out cdclk from the hardware, we have it already
cached in dev_priv.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1448893432-6978-7-git-send-email-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_panel.c

@@ -1345,7 +1345,7 @@ static u32 i9xx_hz_to_pwm(struct intel_connector *connector, u32 pwm_freq_hz)
 	if (IS_PINEVIEW(dev))
 		clock = MHz(intel_hrawclk(dev));
 	else
-		clock = 1000 * dev_priv->display.get_display_clock_speed(dev);
+		clock = 1000 * dev_priv->cdclk_freq;
 
 	return clock / (pwm_freq_hz * 32);
 }
@@ -1364,7 +1364,7 @@ static u32 i965_hz_to_pwm(struct intel_connector *connector, u32 pwm_freq_hz)
 	if (IS_G4X(dev_priv))
 		clock = MHz(intel_hrawclk(dev));
 	else
-		clock = 1000 * dev_priv->display.get_display_clock_speed(dev);
+		clock = 1000 * dev_priv->cdclk_freq;
 
 	return clock / (pwm_freq_hz * 128);
 }