drm/i915/perf: enable perf support on ICL

No significant changes from either context offsets, nor report
formats, nor register whitelist.

v2: Also drop slice/unslice clock ratio changes (Matt)

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180326133949.12469-3-lionel.g.landwerlin@intel.com