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

drivers/gpu/drm/i915/Kconfig.debug

@@ -29,6 +29,7 @@ config DRM_I915_DEBUG
         select DRM_VGEM # used by igt/prime_vgem (dmabuf interop checks)
         select DRM_DEBUG_MM if DRM=y
 	select DRM_DEBUG_SELFTEST
+	select DMABUF_SELFTESTS
 	select SW_SYNC # signaling validation framework (igt/syncobj*)
 	select DRM_I915_SW_FENCE_DEBUG_OBJECTS
 	select DRM_I915_SELFTEST