drm/panel: ili9322: move bus_flags to get_modes()

To prepare the driver to receive drm_connector only in the get_modes()
callback, move bus_flags handling to ili9322_get_modes().

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190804201637.1240-11-sam@ravnborg.org