zink: Always fill external_only in zink_query_dmabuf_modifiers

zink: Always fill external_only in zink_query_dmabuf_modifiers

Fix piglit.spec@ext_image_dma_buf_import@ext_image_dma_buf_import-modifiers
randomly skipping some tests as external_only content was never initialized.

Cc: mesa-stable
Reviewed-by: default avatarMike Blumenkrantz <michael.blumenkrantz@gmail.com>
Signed-off-by: Mary Guillemard <mary.guillemard@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/26305>
(cherry picked from commit b6d828576e1d561ad49c01f19c1cb5ba39e3832a)