drm/panel: raydium-rm67200: Add missing drm_display_mode flags

Add missing drm_display_mode DRM_MODE_FLAG_NVSYNC | DRM_MODE_FLAG_NHSYNC
flags. Those are used by various bridges(e.g. dw-mipi-dsi) in the
pipeline to correctly configure its sync signals polarity.

Tested on rk3568/rk3576/rk3588 EVB.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250618080955.691048-1-andyshrk@163.com