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>