libweston: Damage the output after the output has been added

Rather than damaging the output before the output has been added with
weston_compositor_add_output, do that afterwards as to avoid scheduling
a repaint for that output *before* actually adding the output.

This would avoid the awkward case where we attempt to set initial power
state to normal, but we can't apply it at that stage.

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