drm/amd/display: Changed pipe split policy to allow for multi-display pipe split

[WHY]
Current implementation of pipe split policy prevents pipe split with
multiple displays connected, which caused the MCLK speed to be stuck at
max

[HOW]
Changed the pipe split policies so that pipe split is allowed for
multi-display configurations

Bug: drm/amd#1522
Bug: drm/amd#1709
Bug: drm/amd#1655
Bug: drm/amd#1403



Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Angus Wang <angus.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>