drm/i915/hsw: Use the DPLL ID when calculating DPLL clock

Instead of converting DPLL ID to CLK_SEL to identify the DPLL use the
DPLL ID directly for this.

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/20200226203455.23032-6-imre.deak@intel.com