drm/amd/display: Switch to drm_atomic_helper_wait_for_flip_done

This new helper function is advised to be used for drviers that
use the nonblocking commit tracking support instead of
drm_atomic_helper_wait_for_vblanks.

Signed-off-by: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Reviewed-and-Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>