drm/i915: Add a live dmabuf selftest

Though we have good coverage of our dmabuf interface through the mock
tests, we also want to check the heavy module unload paths of the live
i915 driver.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213171558.20942-26-chris@chris-wilson.co.uk