drm/amd/display: Add comments to v_total calculation and drop legacy TODO

drm/amd/display: Add comments to v_total calculation and drop legacy TODO

[WHY & HOW]
This commit just adds some simple comments to help understand the
calculation of V total duration for Freesync. Also, remove a legacy TODO
comment from link service type.

Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>