drm/amd/display: Take FEC Overhead into Timeslot Calculation

8b/10b encoding needs to add 3% fec overhead into the pbn.
In the Synapcis Cascaded MST hub, the first stage MST branch device
needs the information to determine the timeslot count for the
second stage MST branch device. Missing this overhead will leads to
insufficient timeslot allocation.

Cc: stable@vger.kernel.org
Cc: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>