drm/amdgpu: add ring flag for no user submissions

This would be set by IPs which only accept submissions
from the kernel, not userspace, such as when kernel
queues are disabled. Don't expose the rings to userspace
and reject any submissions in the CS IOCTL.

v2: fix error code (Alex)

Reviewed-by: Sunil <Khatri&lt;sunil.khatri@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>