drm/i915/perf: refactor perf setup

Gen8/9 aren't very different and we can merge some of this code.

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/20171110190845.32574-4-lionel.g.landwerlin@intel.com