drm/amdgpu: add comment to HW_IP_VCN_ENC type

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

Link: https://gitlab.freedesktop.org/mesa/drm/-/merge_requests/245/commits


Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Signed-off-by: Ruijing Dong <ruijing.dong@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>