drm/i915/selftests: Make headers self-contained

Include the types used by the headers to they can be compiled
standalone.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200103104516.1757103-2-chris@chris-wilson.co.uk