drm/i915/selftests: fix NULL vs IS_ERR() check in mock_context_barrier()

The mock_context() function returns NULL on error, it doesn't return
error pointers.

Fixes: 85fddf0b

 ("drm/i915: Introduce a context barrier callback")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190321092451.GK2202@kadam