drm/i915/skl: use previous pll hw readout

drm/i915/skl: use previous pll hw readout

By the time skl_ddi_clock_get() is called - and thus
skl_calc_wrpll_link() - we've just got the hw state from the pll
registers. We don't need to read them again: we can rather reuse what
was cached in the dpll_hw_state.

v2: rename state variable to pll_state, make argument const in
    skl_calc_wrpll_link() and remove not useful warning (from Ville)

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190322223751.22089-2-lucas.demarchi@intel.com