drm/i915/skl: Parametrize the DPLL ref clock instead of open-coding it

For clarity keep the SKL DPLL ref clock in a variable instead of
open-coding it. Store the value in kHZ units as done on other platforms.
This allows us in a later patch to keep track of the DPLL ref clock in a
more unified way across all platforms.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200226203455.23032-8-imre.deak@intel.com