drm/i915/perf: add support for Coffeelake GT3

We can enable GT3 as well as GT2.

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/20171110190845.32574-3-lionel.g.landwerlin@intel.com