drm/amd/display: Tidy up mem_input_program_surface_flip_and_addr()

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>