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

Just use the hardcoded tables provided by our spec.

v2: Rebase.
v3: Clarify that 38.4 uses the 19.2 table (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/20180328215803.13835-6-paulo.r.zanoni@intel.com