drm/amdgpu: Check if NBIO funcs are NULL in amdgpu_device_baco_exit

The special case for VM passthrough doesn't check adev->nbio.funcs
before dereferencing it. If GPUs that don't have an NBIO block are
passed through, this leads to a NULL pointer dereference on startup.
Signed-off-by: Friedrich Vock <friedrich.vock@gmx.de>
Fixes: 1bece222 ("drm/amdgpu: Clear doorbell interrupt status for Sienna Cichlid")
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian König <christian.koenig@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

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

@@ -6167,7 +6167,7 @@ int amdgpu_device_baco_exit(struct drm_device *dev)
 	    adev->nbio.funcs->enable_doorbell_interrupt)
 		adev->nbio.funcs->enable_doorbell_interrupt(adev, true);
 
-	if (amdgpu_passthrough(adev) &&
+	if (amdgpu_passthrough(adev) && adev->nbio.funcs &&
 	    adev->nbio.funcs->clear_doorbell_interrupt)
 		adev->nbio.funcs->clear_doorbell_interrupt(adev);