drm/panel: s6e63m0: Add reading functionality

This adds code to send read commands to read a single
byte from the display, in order to perform MTP ID
look-up of the mounted panel on the s6e63m0 controller.
This is needed for proper biasing on the DSI variants.

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/msgid/20200809215104.1830206-4-linus.walleij@linaro.org