desktop: Avoid sending the dimensions for the first output

desktop: Avoid sending the dimensions for the first output

Rather than sending the first available output we have in the system,
use explicitly the output that we specified in the configuration for
that particular appid. If we didn't do that, we would first send the
dimension for the first output, after which we send the dimensions for
the correct output, resulting in a few configure exchanges between the
client and the compositor.

This fixes that by using the correct output from the beginning.

Bug-AGL: SPEC-4520
Signed-off-by: Marius Vlad <marius.vlad@collabora.com>
Change-Id: If796f1ce8ba069d4fd2c73a7bdfd42e94c9e0418