backend-drm: client view leaves remnant in rendered output after moving it to plane

When Weston moves a view that was previously rendered to a DRM plane, a remnant of this view stays visible in the rendered output.

Reproduce the issue:

1. Start Weston with the Desktop shell and the drm-backend
2. Start weston-simple-dmabuf-egl. -> The view is placed on a plane.
3. Move the window behind the top panel (using Super + Mouse movement). -> The view is placed on the renderer, because it is behind the transparent panel.
4. Move the window away from the top panel. -> The view is placed on a plane.

Expected behavior:

The view is only visible once at the new position.

Observed behavior:

The view is visible twice: In the new expected position, and a remnant at the position, where Weston decided to move it to the plane.