glamor: Don't fail in glamor_get_formats if not dmabuf_capable.

If dmabuf_capable is false, because the server "dmabuf_capable"
debug flag isn't set, treat it as successfull query with zero
returned formats, instead of failure.

This allows the servers cache_formats_and_modifiers() function
to cache the fact that formats are not supported during the
current server generation, instead of pointless retesting at
every invocation.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>