compositor: set surface->plane from destroyed plane to NULL

In drm backend, the cursor_surface->plane point to
drm_output->cursor_plane.when this output is removed,
drm_output->cursor_plane is destroyed, butcursor_surface->plane
still point to destroyed plane. So once mouse move to this
cursor_surface and system will repaint this cursor_surface,
segment fault will generate in weston_surface_damage_below() function.

V2:
-set surface->plane to NULL whose plane point to unplugged output,
 then change weston_surface_damage_below() to do nothing if
 surface->plane is NULL (Kristian)
-set surface->plane to NULL in weston_surface_unmap(),
 so that all surfaces that have a non-NULL plane pointer wil be
 on compositor->surface_list (Kristian).

bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=69777

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
3 files changed