drm/panel: s6e63m0: Support 3WIRE protocol

The panel can be connected using 3WIRE, then it is
however necessary that the flag SPI_3WIRE is preserved
on the device, as we set this from generic device tree
parsing code (or similar). Just |= the SPI mode.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Stephan Gerhold <stephan@gerhold.net>
Cc: Paweł Chmiel <pawel.mikolaj.chmiel@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201110234653.2248594-5-linus.walleij@linaro.org