drm/tve200: Clean up panel bridging

This makes use of the drm_simple_display_pipe_attach_bridge()
call and removes the two calls removing the bridge, which were
erroneous: they unregister the bridge which is not what
we want, we just want to unreference it and that is already
handled by the core.

Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20170910220801.28588-1-linus.walleij@linaro.org