drm/amd/display: Don't allow FPO if no planes

In DCN32/321 FPO uses per-pipe P-State force. If there is no plane, then
then HUBP is power gated, in which case any programming in HUBP has no
effect and the pipe is always asserting P-State allow. This is contrary
to what we want to happen for FPO (FW should moderate the P-State
assertion), so block FPO if there's no plane for the FPO pipe.

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Reviewed-by: Samson Tam <samson.tam@amd.com>
Reviewed-by: Chaitanya Dhere <chaitanya.dhere@amd.com>
Signed-off-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>