drm/amdgpu: mark GC 9.4.3 experimental for now

Mark as experimental for now until we get closer to production
to avoid possible undesireable behavior when mixing newer
boards with older kernels.

Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>