drm/i915/gvt: i915_gem_object_create() returns an error pointer

On failure from i915_gem_object_create(), we need to check for an error
pointer not NULL.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>