drm/amdgpu: remove usec timeout loop from IB tests

We already waited for the fence, so waiting for the registers
is completely pointless and just copy & pasted from the ring test.

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