drm/i915/icp: Get/set proper Raw clock frequency on ICP

Add register definitions for setting the rawclock.
Set the numerator,denominator and divider values.

v2: Simplify the commit message. Simplify the math.
Add  register bits for numerator. (Paulo)
v3 (from Paulo): coding style bikesheds.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180111180010.24357-5-paulo.r.zanoni@intel.com