drm: Restrict stackdepot usage to builtin drm.ko

I misread the kbuild result thinking that we had missed the include
(which we had for completeness anyway), what kbuild was actually warning
me about was that depot_save_stack was not exported.

Temporarily fix this by only selecting STACKDEPOT iff drm.ko is builtin

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Fixes: 5705670d

 ("drm: Track drm_mm allocators and show leaks on shutdown")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161108131917.6253-2-chris@chris-wilson.co.uk