drm/amd/display: un-block 8k with single dimm

[Why]
Driver doesn't validate multi-display with scaling when OS calls DDI.
This behavior causes the validated result to be a mismatch
between some automated test cases.
To address this issue, some workaround was added that caused issues in 8k.

[How]
Since the origin issue had been root caused,
revert the previous workaround and unblock 8k with a single dimm.

Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@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>