drm/mediatek: Clean dangling pointer on bind error path

In the Linux kernel, the following vulnerability has been resolved:

drm/mediatek: Clean dangling pointer on bind error path

mtk_drm_bind() can fail, in which case drm_dev_put() is called,

destroying the drm_device object. However a pointer to it was still

being held in the private object, and that pointer would be passed along

to DRM in mtk_drm_sys_prepare() if a suspend were triggered at that

point, resulting in a panic. Clean the pointer when destroying the

object in the error path to prevent this from happening.