drm/amd/display: Fix incorrect DML output_bpp value

[Why]
The output_bpp pipe parameter was assigned an incorrect value
(color depth enum), and subsequently used to overwrite
the OutputBpp parameter calculated by DML. Seems like this had no
effect except with DSC enabled, which would make DML produce
bad outputs.

[How]
Removed assignment to OutputBpp, fixed output_bpp assignment,
and properly set ForcedOutputLinkBPP instead of OutputBpp.
Also removed condition in DML that prevented forcing of
Output BPP with DSC enabled.

Signed-off-by: Ilya Bakoulin <Ilya.Bakoulin@amd.com>
Reviewed-by: Dmytro Laktyushkin <Dmytro.Laktyushkin@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>