drm/i915/icl: use previous pll hw readout

drm/i915/icl: use previous pll hw readout

By the time icl_ddi_clock_get() is called 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.

While at it, s/refclk/ref_clock/ just to be consistent with the name
used in code nearby.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190322223751.22089-5-lucas.demarchi@intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>