drm/panel: st7703: Add support for Xingbangda XBD599

Xingbangda XBD599 is a 5.99" 720x1440 MIPI-DSI LCD panel used in
PinePhone. Add support for it.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
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/20200701162928.1638874-10-megous@megous.com