i915/gem_softpin: Ensure gpu is idle when we use manual placement

For separate execution there is no risk we may reserve offset which is
still in use by some job. But for running bunch of allocator tests
which may be still on gpu such risk exists. To avoid failures on
grouped execution ensure for 'nopin' subtests gpu is idle first.

Signed-off-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Reviewed-by: Kamil Konieczny <kamil.konieczny@linux.intel.com>