drm/i915: set the correct eDP aux channel clock divider on DDI

The cdclk frequency is not always the same, so the value here should
be adjusted to match it.

Version 2: call intel_ddi_get_cdclk_freq instead of reading
CDCLK_FREQ, because the register is just for earlier HW steppings.

Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_ddi.c

@@ -1239,7 +1239,7 @@ void intel_disable_ddi(struct intel_encoder *encoder)
 	/* This will be needed in the future, so leave it here for now */
 }
 
-static int intel_ddi_get_cdclk_freq(struct drm_i915_private *dev_priv)
+int intel_ddi_get_cdclk_freq(struct drm_i915_private *dev_priv)
 {
 	if (I915_READ(HSW_FUSE_STRAP) & HSW_CDCLK_LIMIT)
 		return 450;