drm/amd/display: Wait for VBLANK during pipe programming

drm/amd/display: Wait for VBLANK during pipe programming

[Description]
- Wait for vblank during front end programming
  for global sync to ensure all double buffer
  updates take.
- This prevents underflow in some cases.

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>