drm: panel: simple: add missing platform_driver_unregister() in panel_simple_init

Add the missing platform_driver_unregister() before return
from panel_simple_init in the error handling case when failed
to register panel_simple_dsi_driver with CONFIG_DRM_MIPI_DSI
enabled.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201031011856.137307-1-miaoqinglang@huawei.com