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>