compositor-drm: Forgot to use mode variable when using render only mode of composition

In patch 5d767416 we simplified a bit the way in which the
compositing mode was being printed with the purpose to improve
weston-debug.  It seems we forgot to use the mode when RENDER-only mode
is being used, so this patch fixes that.

Signed-off-by: Marius Vlad <marius.vlad0@gmail.com>