drm/panel: s6e63m0: Implement reading from panel

This code was found in the Samsung vendor tree for the
Samsung GT-I9070 mobile phone. Let's support reading before
we implement the 3WIRE protocol for both reading and
writing.

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/20201110234653.2248594-3-linus.walleij@linaro.org