backend-pipewire: Move region initialization at the start

Failure to do so, might cause a crash if the output repaint happens
before the pipewire pipeline started -- calling pixman_region32_fini on
a uninitialized region.

Fixes 2abe4efc, "libweston/backends: Move damage flush into backends"

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