linux-dmabuf: clarify DRM_FORMAT_MOD_INVALID

DRM_FORMAT_MOD_INVALID means to derive the modifier from the dmabuf.
It provides legacy support and makes it easier to replace wl_drm.

v3: DRM_FORMAT_MOD_INVALID must be advertised to be supported (which
    requires a version bump)
v4: no version bump, but a note for now

Signed-off-by: Chia-I Wu <olvaffe@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Stone <daniels@collabora.com>