xf86drm: drmGetDevice2: error out if the fd has unknown subsys

Currently one can open() any /dev node. If it's unknown
drmParseSubsystemType() will return an error.

Track that and bail as needed.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Robert Foss <robert.foss@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>