Render only DRM devices

[Christian König]

Hi guys,

we already have quite a bunch of devices which are essentially render only and don't expose any connectors or more general display functionality.

Just recently I ran into a case where an older X/DDX combination caused problems for such a device so I looked a bit into the possibility to allow drivers to disable the primary node and only expose the render node.

It turned out that this effectively hides the device from X, but OpenGL and Vulkan can still use it perfectly fine.

The only crux is that this is checked so early in the initialization that drivers don't have an opportunity to update their dev->driver_features. So we will always need a separate drm_driver structure for render only devices.

Please review and comment,
Christian.