drm/amd/display: Fix fclk idle state

[Why]
The earlier change 'Fix 6x4K displays' led to fclk value
idling at higher DPM level.

[How]
Apply the fix only to respective multi-display configuration.

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>