drm/i915: Actually read out DPLL0 vco on skl from hardware

Currently we're trying to guess which lcpll vco frequency is used
use based on the cdclk. That doesn't work for cdclk==540 since
both vco frequencies can generate a 540 Mhz output. Let's stop
guessing and just read the actual vco frequency from the
hardware.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463172100-24715-6-git-send-email-ville.syrjala@linux.intel.com


Reviewed-by: Imre Deak <imre.deak@intel.com>