drm/amdgpu/vce3: add support for third vce ring

Not of much use at the moment (we don't really use
the second ring either), but may be useful later.

Reviewed-by: JimQu <Jim.Qu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>