drm: rcar-du: Move need_restart flag to private state

This moves the restart operations to occur *after* the following writes,
where as they were before:

du_write: 0x00011024 0x00000003
du_write: 0x00030010 0x00000800
rcar_du_vsp_atomic_flush
du_write: 0x00030000 0x00000000
du_write: 0x00000000 0x00000180

WIP
5 files changed