drm/i915: use 125MHz reference clock for PCH attached eDP

Fix the test so we don't try to use the 450MHz refclk on PCH attached
eDP.

References:
  https://bugs.freedesktop.org/show_bug.cgi?id=29141

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_dp.c

@@ -246,8 +246,11 @@ intel_dp_aux_ch(struct intel_dp *intel_dp,
 	/* The clock divider is based off the hrawclk,
 	 * and would like to run at 2MHz. So, take the
 	 * hrawclk value and divide by 2 and use that
+	 *
+	 * Note that PCH attached eDP panels should use a 125MHz input
+	 * clock divider.
 	 */
-	if (IS_eDP(intel_dp)) {
+	if (IS_eDP(intel_dp) && !IS_PCH_eDP(intel_dp)) {
 		if (IS_GEN6(dev))
 			aux_clock_divider = 200; /* SNB eDP input clock at 400Mhz */
 		else