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: 50f0033d1a ("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

drivers/gpu/drm/Makefile

@@ -38,7 +38,7 @@ drm_kms_helper-$(CONFIG_DRM_KMS_CMA_HELPER) += drm_fb_cma_helper.o
 drm_kms_helper-$(CONFIG_DRM_DP_AUX_CHARDEV) += drm_dp_aux_dev.o
 
 obj-$(CONFIG_DRM_KMS_HELPER) += drm_kms_helper.o
-obj-$(CONFIG_DRM_DEBUG_MM_SELFTEST) += selftests/test-drm_mm.o
+obj-$(CONFIG_DRM_DEBUG_MM_SELFTEST) += selftests/
 
 CFLAGS_drm_trace_points.o := -I$(src)