drm/i915/icl: compute the TBT PLL registers

Use the hardcoded tables provided by our spec.

v2:
  - SSC stays disabled.
  - Use intel_port_is_tc().

Cc: Anusha Srivatsa <anusha.srivatsa@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180711215909.23945-2-paulo.r.zanoni@intel.com