drm/amd/display: constify a few dc_surface_update fields

Signed-off-by: Jun Lei <Jun.Lei@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>