drm/amd/display: Allow update stream without surface

Passing NULL as surfaceUpdates to dc_update_surfaces_and_stream now
updates just the stream. Code that is dependent on srf_updates was moved
below the NULL check.

Signed-off-by: Corbin McElhanney <corbin.mcelhanney@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>