drm/i915/selftests: prefer the create_user helper

No need to hand roll the set_placements stuff, now that we have a helper
for this.

v2: add back the -ENODEV checking since it's possible for stolen to be
probed, and yet still be non-functional

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20210729094731.1953091-1-matthew.auld@intel.com