drm/i915: rename CNP_RAWCLK_FRAC to CNP_RAWCLK_DEN

Although CNP names this field "Counter Fraction", what we write to the
register is really the denominator for the fractional part of the
divider, not the fractional part (and the field description even says
that). The ICP spec renamed the field to "Counter Fraction
Denominator", which makes a lot more sense. Use the more complete ICL
naming because we will merge the CNP and ICP functions into a single
one, which will introduce the concept of the numerator. That will make
a lot more sense when you read the "num/frac = den" calculation.

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181112232313.26373-2-paulo.r.zanoni@intel.com