drm/amd/display: Correct DML calculation to follow HW SPEC

[Why]
In 2560x1600@240p eDP panel, driver use lowest voltage level
to play 1080p video cause underflow. According to HW SPEC,
the senario should use high voltage level.

[How]
ChromaPre value is zero when bandwidth validation.
Correct ChromaPre calculation.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Paul Hsieh <Paul.Hsieh@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>