backend-drm: Fix visibility calculation

Between assign_planes() and pnode_update_late(), the pnode's plane may
not yet be up to date. This leads to the visible region being incorrectly
calculated for paint nodes beneath a paint node that changes planes. Their
visible regions will still contain a cut out for the node that no longer
occludes them.

However, we place damage on nodes beneath a node that changes planes in
order to redraw the region beneath a node that moves from the primary to
non-primary plane.

The gl-renderer clips to a paint node's visible region when rendering it,
so this accidental cut-out masks away all the damage and leaves us with
a mess.

Fix this by using the correct plane in the visibility calculation.

Fixes #821

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>