gem_ring_sync_copy: Add a ring to ring synchronization test

gem_ring_sync_copy: Add a ring to ring synchronization test

The goal of this test is to ensure that we respect inter ring
dependencies. A more detailed description of what it tests is in a
comment.

The tests relies on having a blit function for the ring, so is currently
only checking synchronization between the render and blitter ring.

v2: Actually create an inter-ring dependency by making the first copy on
ring2 and the second on ring2, not both on ring2.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>