drm/i915/icl: compute the combo PHY (DPLL) HDMI registers

HDMI mode DPLL programming on ICL is the same as CNL, so just reuse
the CNL code.

v2:
 - Properly detect HDMI crtcs.
 - Rebase after changes to the cnl function (clock * 1000).
v3:
 - Add a comment to clarify why we treat 38.4 as 19.2 (James).

Reviewed-by: James Ausmus <james.ausmus@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180328215803.13835-5-paulo.r.zanoni@intel.com