drm/i915: Simplify vlv/chv rc6 residency calculation

drm/i915: Simplify vlv/chv rc6 residency calculation

We have the czclk frequency in dev_priv now, so let's just use it
when converting the rc6 counters to milliseconds. This eliminates
a bunch of hairy code that essentially tries to extract the czclk
frequency using yet another method.

v2: Fix typos in commit message (Imre)

Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>