drm/i915: Select DMABUF_SELFTESTS for the default i915.ko debug build

Include the DMABUF_SELFTESTS as part of the standard build for IGT, so
that they can be run by igt/dmabuf

Testcase: igt/dmabuf
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tomi Sarvela <tomi.p.sarvela@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190819171900.4501-1-chris@chris-wilson.co.uk