drm/mm: Allow CONFIG_DRM_MM_DEBUG with DRM=m

The original rationale for
commit cd456f8d

 ("drm: Restrict stackdepot usage to builtin drm.ko")
was that depot_save_stack() (which is what we used back then)
wasn't exported. stack_depot_save() (which is what we use now) is
exported however, so relax the dependency allow CONFIG_DRM_MM_DEBUG
with DRM=m.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20231215111129.9559-1-ville.syrjala@linux.intel.com
Acked-by: Javier Martinez Canillas <javierm@redhat.com>