drm/i915/icl: compute the MG PLL registers

This implements the "MG PLL Programming" sequence from our spec. The
biggest problem was that the spec assumes real numbers, so we had to
adjust some numbers and calculations due to the fact that the Kernel
prefers to deal with integers.

I recommend grabbing some coffee, a pen and paper before reviewing
this patch.

v2:
 - Correctly identify DP encoders after upstream change.
 - Small checkpatch issues.
 - Rebase.
v3:
 - Try to impove the comment on the tdc_targetcnt calculation based on
   Manasi's feedback (Manasi).
 - Rebase.

Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180328215803.13835-7-paulo.r.zanoni@intel.com