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: ce801e5d ("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>