drm: Restrict stackdepot usage to builtin drm.ko

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 <[email protected]>
Fixes: 5705670d0463 ("drm: Track drm_mm allocators and show leaks on shutdown")
Signed-off-by: Chris Wilson <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]