drm/amd/display: Fix pbn to kbps Conversion

drm/amd/display: Fix pbn to kbps Conversion

[Why]
Existing routine has two conversion sequence,
pbn_to_kbps and kbps_to_pbn with margin.
Non of those has without-margin calculation.

kbps_to_pbn with margin conversion includes
fec overhead which has already been included in
pbn_div calculation with 0.994 factor considered.
It is a double counted fec overhead factor that causes
potential bw loss.

[How]
Add without-margin calculation.
Fix fec overhead double counted issue.

Closes: https://gitlab.freedesktop.org/drm/amd/-/issues/3735
Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Signed-off-by: Ivan Lipski <ivan.lipski@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>