drm/i915/selftests: Exercise adjusting rpcs over all render-class engines

Iterate over all user-accessible render engines when checking whether
they can be adjusted for sseu.

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/20191027225808.19437-2-chris@chris-wilson.co.uk