drm/tidss: Use drm_for_each_bridge_in_chain()

Replace the manual encoder->bridge_chain walk with the
drm_for_each_bridge_in_chain() macro. Drivers should not touch the
bridge_chain field directly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Jyri Sarha <jsarha@ti.com>
Tested-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Jyri Sarha <jsarha@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200222110718.26272-1-laurent.pinchart@ideasonboard.com