drm: rcar-du: Fix build failure

Commit 75a07f39 ("drm: rcar-du: Zero-out sg_tables when duplicating
plane state") introduced a reference to the alpha field of struct
rcar_du_vsp_plane_state that got removed in commit 301a9b8d
("drm/rcar-du: Convert to the new generic alpha property"). The issue
stems from the merge of the two commits through separate branches and
breaks compilation of the driver. Fix it.

Fixes: 75a07f39 ("drm: rcar-du: Zero-out sg_tables when duplicating plane state")
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180515174752.28954-1-laurent.pinchart+renesas@ideasonboard.com