drm/amd/display: reset stream slice count for new ODM policy

[why]
ODM combine could prevent us from supporting more planes
we will reset ODM slice count back to 1 when all planes have
been removed to maximize the amount of planes supported when
new planes are added.

[how]
reset ODM slice count when all planes are removed.

Reviewed-by: Aric Cyr <aric.cyr@amd.com>
Acked-by: Wayne Lin <wayne.lin@amd.com>
Signed-off-by: Wenjing Liu <wenjing.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>