drm/panel: s6e63m0: Add some explanations

The SPI DCS code was a bit hard to understand as the
device accepts 9-bit transfers packed into 16-bit words
with the most significant bit in bit 9 of the
16-bit word. Add some clarifying comments.

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/20201110234653.2248594-4-linus.walleij@linaro.org