drm/amd/display: Block zstate when more than one plane enabled

[Why]
Currently Z10 save/restore only covers 1 plane case. If we enter
Z10 with 2 planes enabled, 1 of the 2 planes will not be restored.
causing black screen.

[How]
Disable Zstate when more than 1 plane is enabled. Right now Z9 enable
but Z10 disable is not a supported config, so we disable both.

Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Reviewed-by: Nevenko Stupar <Nevenko.Stupar@amd.com>
Acked-by: Agustin Gutierrez <agustin.gutierrez@amd.com>
Signed-off-by: Eric Yang <Eric.Yang2@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>