kmscon: enable --all-gpus by default

kmscon: enable --all-gpus by default

Users with multiple GPUs that cannot be used simultaneously should pass
--no-all-gpus if they encounter problems or use --video-devices
explicitly. However, we cannot rely on PRIMARY flags being set for all
kinds of setups. Instead, default behavior should be using all available
GPUs unconditionally (except the safe DRM_BACKED and fbdev PRIMARY logic).

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>