drm: Fix O= out-of-tree builds for selftests

drm: Fix O= out-of-tree builds for selftests

Kbuild really doesn't like non-recursive Makefiles, but they do work
as long as you build without O=

Reported-by: kbuild test robot <[email protected]>
Fixes: 50f0033d1a0f ("drm: Add some kselftests for the DRM range manager (struct drm_mm)")
Cc: Chris Wilson <[email protected]>
Cc: Christian König <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]