libweston: don't reset the plane for views from other outputs

The paint_node_z_order_list contains all views, not just the ones visible on the
current output. So all views are moved to the primary plane when one output
does not support planes.

This will be relevant with multiple backends: When an output without plane
support is rendered then the views of all other outputs are removed from
the current planes and the corresponding outputs will be repainted
unnecessarily.

So only reset the plane if the view is actually on the current output.

Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>