drm/amdgpu/userq: enable support for queue priorities

Enable users to create queues at different priority levels.
The highest level is restricted to drm master.

Reviewed-by: Sunil Khatri <sunil.khatri@amd.com>
Reviewed-by: Jesse.Zhang <Jesse.zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>