drm/mcde: Break out DSI set-up routine

To be able to support DPI without messing things up we
first break out the DSI set-up to a separate function.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Stephan Gerhold <stephan@gerhold.net>
Cc: phone-devel@vger.kernel.org
Cc: upstreaming@lists.sr.ht
Link: https://patchwork.freedesktop.org/patch/msgid/20201112142925.2571179-1-linus.walleij@linaro.org