drm/i915/cnl: only divide up base frequency with crystal source

We apply this logic to Gen9 as well. We didn't notice this issue as
most part we've encountered so far only use the crystal as source for
their timestamp registers.

Fixes: dab91783 ("drm/i915: expose command stream timestamp frequency to userspace")
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171113233455.12085-5-lionel.g.landwerlin@intel.com