drm/amdgpu: partially revert VCN IP block instancing support

This partially reverts the VCN IP block rework.  There
are too many corner cases and chances for regressions.

While this aligned better with the original design, years
of hardware has used the old pattern.  Best to stick with
it at this point.

Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>