drm/i915/selftests: Be explicit in ERR_PTR handling

When setting up a full GGTT, we expect the next insert to fail with
-ENOSPC. Simplify the use of ERR_PTR to not confuse either the reader or
smatch.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
References: f40a7b75 ("drm/i915: Initial selftests for exercising eviction")
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/20191120093302.3723715-8-chris@chris-wilson.co.uk