drm/panel: st7703: Prepare for supporting multiple panels

Parametrize the driver so that it can support more panels based
on st7703 controller.

Signed-off-by: Ondrej Jirman <megous@megous.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200701162928.1638874-7-megous@megous.com