drm/i915/selftests: Pin the mock kernel context

The driver assumes that the kernel context is pinned and always
available for use from any process or atomic context. Make it so for
selftesting as well!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180523142346.27953-1-chris@chris-wilson.co.uk