drm/amd/display: Fix MST BW calculation Regression

[Why & How]
Revert commit 8b2cb32c
("drm/amd/display: FEC overhead should be checked once for mst slot nums")
Because causes bw calculation regression

Cc: mario.limonciello@amd.com
Cc: alexander.deucher@amd.com
Reported-by:  <jirislaby@kernel.org>
Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3495
Closes: https://bugzilla.suse.com/show_bug.cgi?id=1228093


Reviewed-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Signed-off-by: Tom Chung <chiahsuan.chung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>