drm/amdgpu: fix unload driver issue for virtual display

Virtual display doesn't allocate amdgpu_encoder when initializing,
so will get invaild pointer if try to free amdgpu_encoder when
unloading driver.

Signed-off-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/amd/amdgpu/dce_virtual.c

@@ -627,11 +627,8 @@ static const struct drm_encoder_helper_funcs dce_virtual_encoder_helper_funcs =
 
 static void dce_virtual_encoder_destroy(struct drm_encoder *encoder)
 {
-	struct amdgpu_encoder *amdgpu_encoder = to_amdgpu_encoder(encoder);
-
-	kfree(amdgpu_encoder->enc_priv);
 	drm_encoder_cleanup(encoder);
-	kfree(amdgpu_encoder);
+	kfree(encoder);
 }
 
 static const struct drm_encoder_funcs dce_virtual_encoder_funcs = {