drm/amd/display: Add htmldocs description for fused_io interface

[Why]
htmldocs build warning: "Function parameter or struct member 'fused_io'
not described in 'amdgpu_display_manager'".

[How]
Add missing description.

Fixes: ce801e5d6c ("drm/amd/display: HDCP Locality check using DMUB Fused IO")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Roman Li <Roman.Li@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.h

@@ -639,6 +639,11 @@ struct amdgpu_display_manager {
 	 */
 	struct amdgpu_i2c_adapter *oem_i2c;
 
+	/**
+	 * @fused_io:
+	 *
+	 * dmub fused io interface
+	 */
 	struct fused_io_sync {
 		struct completion replied;
 		char reply_data[0x40];  // Cannot include dmub_cmd here