drm/amd/display: fix underflow in some two display subvp/non-subvp configs

drm/amd/display: fix underflow in some two display subvp/non-subvp configs

[Why]
In two display configuration, switching between subvp and non-subvp
 may cause underflow because it moves an existing pipe between
 displays

[How]
Create helper function for applying pipe split flags
Apply pipe split flags prior to deciding on subvp
During subvp check, do not merge pipes, so it can retain previous
 pipe configuration
Add check for prev odm pipe in subvp check
For single display subvp case, use same odm policy for phantom pipes
 as main subvp pipe

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Acked-by: Roman Li <roman.li@amd.com>
Signed-off-by: Samson Tam <samson.tam@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>