drm: rcar-du: Zero-out sg_tables when duplicating plane state

drm: rcar-du: Zero-out sg_tables when duplicating plane state

The state structure for VSP-backed planes, rcar_du_vsp_plane_state,
contains sg tables that track framebuffer mapping performed in the
.prepare_fb() operation to unmap them in .cleanup_fb(). The tables are
incorrectly copied when duplicating state, which can result :

Zero-out sg_tables in original plane, effectively introducing move
semantic. Seems, this fixes issue with double-free,
when rcar_du_vsp_plane_cleanup_fb() freed the same sg_table
both in original plane and in the copy.

Reported-by: Volodymyr Babchuk <vlad.babchuk@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>