renderer: Move dmabuf setup into renderer init

We've forgotten to set this up in some backends, so let's just do it in
weston_compositor_init_renderer().

The headless backend used to fail out if linux_dmabuf_setup() failed, but
had no reason to do so, so just remove that to make the code common.

Suggested by cwabbott on irc.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>