[media] v4l: Add a media_device pointer to the v4l2_device structure
The pointer will later be used to register/unregister media entities
when registering/unregistering a v4l2_subdev or a video_device.
With the introduction of media devices, device drivers need to store a
pointer to a driver-specific structure in the device's drvdata.
v4l2_device can't claim ownership of the drvdata anymore.
To maintain compatibility with drivers that rely on v4l2_device storing
a pointer to itself in the device's drvdata, v4l2_device_register() will
keep doing so if the drvdata is NULL.