drm/i915/selftests/perf: watch out for stolen objects

Stolen memory is allocated at creation, returning -ENOSPC if we run out
space.

Closes: https://gitlab.freedesktop.org/drm/intel/issues/1424


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/20200323110301.38806-1-matthew.auld@intel.com