drm/amd/display: Add output bitrate to DML calculations

[why]
Output bitrate was mistakenly left out, causing corruption on some
DSC low resolution (such as 800x600) modes.

Signed-off-by: Nikola Cornij <nikola.cornij@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Bhawanpreet Lakha <Bhawanpreet.Lakha@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>