drm/amd/display: Do not trigger timing sync for phantom pipes

[Why&How]
Doing timing sync seqence for phantom pipes will not go through since
they are not fully programmed like normal pipes. Skip the sequence on
such pipes

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Alvin Lee <Alvin.Lee2@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>