Document missing parameter.

Cc: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Fixes: 8889a13f99e5 ("drm/amd/display: Add some extra kernel doc to amdgpu_dm")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>