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