drm/i915: Allow enable/disable of DPLL0 around cdclk changes on SKL

drm/i915: Allow enable/disable of DPLL0 around cdclk changes on SKL

In case we originally guessed wrong which lcpll vco frequency to use,
we will need to shut down the pll and restart it when reprogamming the
cdclk.

This also allows us to track the actual vco frequency in dev_priv
instead of just a guess.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463172100-24715-8-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Imre Deak <imre.deak@intel.com>