drm/amd/display: Revert changes for MPO underflow

[WHY]
The previous changes for fixing MPO underflow with multiple
display connected caused a regression where the machine runs
into a hang when doing multiple driver pnp with multiple displays
connected

[HOW]
Reverted offending change

Reviewed-by: Martin Leung <Martin.Leung@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Angus Wang <angus.wang@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>