drm/i915: Select STACKDEPOT for DRM_I915_DEBUG

select in Kconfig isn't recursive, we need to select the stuff our
selects select, too. Fix that.

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180320125009.2305-1-daniel.vetter@ffwll.ch