drm/i915/cnl: use previous pll hw readout

drm/i915/cnl: use previous pll hw readout

By the time cnl_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.

This also affects the code for ICL since it partially reuses the CNL
code. However the more intricate part on ICL is left for another patch.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190322223751.22089-4-lucas.demarchi@intel.com
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>