backend-drm: document that we allow to import wl_drm buffers to KMS with no modifiers

In commit "backend-drm: do not import dmabuf buffers with no modifiers
to KMS" we've stopped to import dmabuf with no modifiers to KMS.

In this patch we document why we can still import wl_drm buffers with no
modifiers to KMS.

Signed-off-by: Leandro Ribeiro <leandro.ribeiro@collabora.com>