drm: add support for render nodes.

Allow vaGetDisplayDRM() to accept DRM Render-Nodes file descriptors,
thus skipping authentication burdens in that case. This also allows
remote users, connected through ssh for instance, to enjoy hardware
acceleration even if there is a local display opened (user logged in).

Signed-off-by: Andrey Larionov <andrey.larionov@intel.com>
[checked if fd is a render-node, fixed VA_DISPLAY_DRM_RENDERNODES]
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>
tests: try a render-node device by default for VA/DRM displays.

Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>