drm/i915/perf: move perf types to their own header

Following a pattern used throughout the driver.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190909093116.7747-7-lionel.g.landwerlin@intel.com