drm/amdgpu: Simplify amdgpu_lockup_timeout usage.

With introduction of amdgpu_gpu_recovery we don't need any more
to rely on amdgpu_lockup_timeout == 0 for disabling GPU reset.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>