drm/amdgpu: remove duplicated codes

the fence_context and seqno is init in amdgpu_vm_manager_init() &
amdgpu_vmid_mgr_init(). remove the amdgpu_vmid_mgr_init() copy.

Signed-off-by: Flora Cui <Flora.Cui@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>