drm/amdgpu: limit AV1 to the first instance on VCN4 encode

AV1 is only supported on the first instance.
Added vcn_v4_0_enc_find_ib_param() to help search for an IB param.

Signed-off-by: David (Ming Qiang) Wu <David.Wu3@amd.com>
Reviewed-by: Ruijing Dong <ruijing.dong@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>