tests/amdgpu/vcn: align comments for AMDGPU_HW_IP_VCN_ENC

From VCN4, AMDGPU_HW_IP_VCN_ENC is re-used to support both encoding
and decoding jobs.

Reviewed-by: Boyuan Zhang <Boyuan.Zhang@amd.com>
Signed-off-by: Ruijing Dong <ruijing.dong@amd.com>