drm/amdgpu: fix fence fallback check

Interrupts are notorious unreliable, enable the fallback at
a couple of more places.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>