drm/amdgpu/vcn: switch work handler to be per instance

drm/amdgpu/vcn: switch work handler to be per instance

Have a separate work handler for each VCN instance. This
paves the way for per instance VCN power gating at runtime.

v2: index instances directly on vcn1.0 and 2.0 to make
it clear that they only support a single instance (Lijo)

Reviewed-by: Boyuan Zhang <Boyuan.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>