drm/amd/display: Correct DML calculation to align HW formula

[Why]
In 2560x1440@240p eDP panel, some use cases will enable MPC
combine with RGB MPO then underflow happened. This case is
not allowed from HW formula. 

[How]
Correct eDP, DP and DP2 output bpp calculation to align HW
formula.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@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>