i965: read CS timestamp frequency from the kernel on Gen10+

i965: read CS timestamp frequency from the kernel on Gen10+

We cannot figure this value out of the PCI-id anymore. Let's read it
from the kernel (which computes this from a few registers).

When running on a (upcoming) 4.16-rc1+ kernel, this will fixes piglit
tests on CNL :

    spec@arb_timer_query@query gl_timestamp
    spec@arb_timer_query@timestamp-get
    spec@ext_timer_query@time-elapsed

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>