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/commitsReviewed-by: NChristian König <christian.koenig@amd.com>
Reviewed-by: NLeo Liu <leo.liu@amd.com>
Signed-off-by: NRuijing Dong <ruijing.dong@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>

include/uapi/drm/amdgpu_drm.h

@@ -559,6 +559,10 @@ struct drm_amdgpu_gem_va {
 #define AMDGPU_HW_IP_VCE          4
 #define AMDGPU_HW_IP_UVD_ENC      5
 #define AMDGPU_HW_IP_VCN_DEC      6
+/*
+ * From VCN4, AMDGPU_HW_IP_VCN_ENC is re-used to support
+ * both encoding and decoding jobs.
+ */
 #define AMDGPU_HW_IP_VCN_ENC      7
 #define AMDGPU_HW_IP_VCN_JPEG     8
 #define AMDGPU_HW_IP_NUM          9