drm/i915: add ICP support to cnp_rawclk() and kill icp_rawclk()

I think I'm probably the one who argued in favor of having separate
implementations for both PCHs, but the calculations are actually the
same, the clocks are the same and the only difference is that on ICP
we write the numerator to the register.

I have previously suggested to kill cnp_rawclk() and keep the
icp_rawclk() style, but Ville gave some good arguments that what's in
this patch may be the better choice.

v2: Switch numerator to 1 from 1000 and adjust calculations
    accordingly (Ville).

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
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/20181112232313.26373-3-paulo.r.zanoni@intel.com