libweston: Set default power state at output initialization

Rather than setting the initial power state when adding
it (using weston_compositor_add_output), do that at the initilization
stage.

Reason being that the compositor can set up the output from the start as
FORCED_OFF, before enabling the output, rather than enabling the output
and then turning off the power of the output.

Signed-off-by: marius vlad <marius.vlad@collabora.com>