drm/i915/hsw: Use the read-out WRPLL/SPLL state instead of reading out again

Instead of reading out the WRPLL/SPLL control values from HW, we can use
the DPLL state that was already read out, or swapped-to.

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/20200226203455.23032-13-imre.deak@intel.com