drm/i915/selftests: Allow random array allocation to fail

In the selftests, we don't want to force an oom and would rather
ENOMEM be reported. In this case, we would rather the allocation for the
random array to fail.

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/20171223110407.21402-2-chris@chris-wilson.co.uk