Output: Choose sane default mode

Output: Choose sane default mode

We currently use the first mode in the list if no mode is explicitely specified
by the caller. However, this is most often the lowest available resolution
reported by the kernel.

This patch adds a new pointer to every output which remembers a sane default.
This is currently the first mode reported by the kernel but may later be
optimized to choose a better default mode.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>