drm/amdgpu: track the number of vce rings

Rather than using a hardcoded value.  This allows
different versions to expose more or less rings.

No functional change.

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