drm/amdgpu/vcn: fix idle work handler for VCN 2.5

VCN 2.5 uses the PG callback to enable VCN DPM which is
a global state.  As such, we need to make sure all instances
are in the same state.

v2: switch to a ref count (Lijo)
v3: switch to its own idle work handler
v4: fix logic in DPG handling

Fixes: 4ce4fe27 ("drm/amdgpu/vcn: use per instance callbacks for idle work handler")
Reviewed-by: Boyuan Zhang <Boyuan.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>