drm/i915/tgl: Fix Combo PHY DPLL fractional divider for 38.4MHz ref clock

Apply Display WA #22010492432 for combo PHY PLLs too. This should fix a
problem where the PLL output frequency is slightly off with the current
PLL fractional divider value.

I haven't seen an actual case where this causes a problem, but let's
follow the spec. It's also needed on some EHL platforms, but for that we
also need a way to distinguish the affected EHL SKUs, so I leave that
for a follow-up.

v2:
- Apply the WA at one place when calculating the PLL dividers from the
  frequency and the frequency from the dividers for all the combo PLL
  use cases (DP, HDMI, TBT). (Ville)

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201003001846.1271151-6-imre.deak@intel.com