drm/amd/display: Fixing stream allocation regression

For certain dual display configs that had one display using a 1080p
mode, the DPM level used to drive the configs regressed from DPM 0 to
DPM 3. This was caused by a missing check that should have only limited
the pipe segments on non-phantom pipes. This caused issues with detile
buffer allocation, which dissallow subvp from being used

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Reviewed-by: Dillon Varone <dillon.varone@amd.com>
Reviewed-by: Martin Leung <martin.leung@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Relja Vojvodic <relja.vojvodic@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>