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 <fengguang.wu@intel.com>
Fixes: 50f0033d

 ("drm: Add some kselftests for the DRM range manager (struct drm_mm)")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Christian König <christian.koenig@amd.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1482918077-30027-1-git-send-email-daniel.vetter@ffwll.ch