drm/amd/amdgpu: Recovery vcn instance iterate.

The previous logic is recording the amount of valid vcn instances
to use them on SRIOV, it is a hard task due to the vcn accessment is
based on the index of the vcn instance.

Check if the vcn instance enabled before do instance init.

Signed-off-by: Peng Ju Zhou <PengJu.Zhou@amd.com>
Reviewed-by: Monk Liu <monk.liu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>