linux-dmabuf-next: don't require a main device to be present

dmabuf-v4 cannot be implemented correctly if the compositor cannot guarantee that a main device is present at all times. A compositor might want to

• work on systems where no drm device is present (but where a device can potentially be added at runtime).
• allow the user to disable all drm devices.
• handle the case where the main device is removed from the system at runtime.
• handle device resets where the device is temporarily removed and later restored by the kernel.

In all of these cases no main device is present temporarily or permanently. Compositors can practically handle this situation by simply violating the spec and not sending a main device. What's the worst that can happen?

In the next version of the protocol, the requirement to send a main device should be removed. Clients can handle this at done time by checking if a main device was sent. They can then either exit(1) or deal with this situation until the compositor advertises a main device.