drm/i915: Restrict DRM_DEBUG_MM automatic selection

DRM_DEBUG_MM is only valid if the DRM.ko is builtin as currently
depot_save_stack is not exported.

Fixes: 5c7fcf2d ("drm/i915: Enable drm_mm debug when enabling DRM_I915_DEBUG")
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/20161108131917.6253-1-chris@chris-wilson.co.uk