drm/amdgpu: drop unused df init callback

It was replaced with the sw_init callback so is no longer
needed.
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/amdgpu.h

@@ -687,7 +687,6 @@ struct amdgpu_nbio_funcs {
 };
 
 struct amdgpu_df_funcs {
-	void (*init)(struct amdgpu_device *adev);
 	void (*sw_init)(struct amdgpu_device *adev);
 	void (*enable_broadcast_mode)(struct amdgpu_device *adev,
 				      bool enable);