drm/i915/icl+: Simplify combo/TBT PLL calculation call-chain

To simplify things, call the combo PHY/TBT PLL calculation functions
directly from the corresponding combo/TypeC PLL get functions, instead of
calling the same calculation functions after having to recheck if the
given PHY is combo or TypeC.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200629185848.20550-2-imre.deak@intel.com