drm/amdgpu: drop old ip definitions for gfxhub and mmhub

The gfxhub and mmhub code are now helpers for gmc rather
than standalone IPs.  When that changes these were left
over.  Remove them.

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