drm/amdgpu: flag renoir as experimental for now

The current code won't likely work on production hw when
it ships so leave it as experimental until it's ready.

Acked-by: Huang Rui <ray.huang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>