drm/i915/selftests: extend coverage to include LMEM huge-pages

Add LMEM objects to list of backends we test for huge-GTT-pages.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20191025153728.23689-5-chris@chris-wilson.co.uk