drm/i915/selftest: also consider non-contiguous objects

In igt_ppgtt_sanity_check we should also exercise the non-contiguous
option for LMEM, since this will give us slightly different sg layouts
and alignment.

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/20201130141809.65330-1-matthew.auld@intel.com