drm/i915/selftests: add sanity selftest for huge-GTT-pages

Now that for all the relevant backends we do randomised testing, we need
to make sure we still sanity check the obvious cases that might blow up,
such that introducing a temporary regression is less likely.  Also
rather than do this for every backend, just limit to our two memory
types: system and local.

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
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/20191025153728.23689-7-chris@chris-wilson.co.uk