drm/amd/display: Clean up submit_channel_request

Signed-off-by: Charlene Liu <charlene.liu@amd.com>
Reviewed-by: Vitaly Prosyak <Vitaly.Prosyak@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>