drm/panel: s6e63m0: Break out SPI transport

This panel can be accessed using both SPI and DSI.

To make it possible to probe and use the device also from
a DSI bus, first break out the SPI support to its own file.

Since all the panel driver does is write DCS commands to
the panel, we pass a DCS write function to probe()
from each subdriver.

We make the Kconfig entry for SPI mode default so all
current users will continue to work.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Stephan Gerhold <stephan@gerhold.net>
Cc: Paweł Chmiel <pawel.mikolaj.chmiel@gmail.com>
Acked-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/384873/