drm/panel: visionox-rm69299: don't unregister DSI device

The DSI device for the panel was registered by the DSI host, so it is an
error to unregister it from the panel driver. Drop the call to
mipi_dsi_device_unregister().

Fixes: c7f66d32 ("drm/panel: add support for rm69299 visionox panel")
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240404-drop-panel-unregister-v1-1-9f56953c5fb9@linaro.org