drm/i915/gen11: add support for reading the timestamp frequency

The only thing that differs here is that the crystal clock freq now
has four possible values.

This patch gets rid of the "Unknown gen, unable to compute..." message
at boot for gen11.

Reviewed-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/20180109232835.11478-18-paulo.r.zanoni@intel.com