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_drv.h

@@ -608,6 +608,7 @@ extern bool intel_ddi_get_hw_state(struct intel_encoder *encoder,
 extern void intel_ddi_mode_set(struct drm_encoder *encoder,
 				struct drm_display_mode *mode,
 				struct drm_display_mode *adjusted_mode);
+extern int intel_ddi_get_cdclk_freq(struct drm_i915_private *dev_priv);
 extern void intel_ddi_pll_init(struct drm_device *dev);
 extern void intel_ddi_enable_pipe_func(struct drm_crtc *crtc);
 extern void intel_ddi_disable_transcoder_func(struct drm_i915_private *dev_priv,