drm/amd/display: Clear phantom stream count and plane count

When dc_state_destruct() was refactored the new phantom_stream_count
and phantom_plane_count members weren't cleared.

Fixes: 012a04b1

 ("drm/amd/display: Refactor phantom resource allocation")
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>