media: ccs-pll: Split off VT subtree calculation

Split off the VT sub clock tree calculation from the rest, into its own
function. Also call the op_pll_fr argument pll_fr, since soon these may
not be OP tree values.

This paves way for additional features in the future such as dual PLL
support.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>