tests/gem_ring_sync_copy: reduce memory usage

tests/gem_ring_sync_copy: reduce memory usage

gem_ring_sync_copy uses a lot of memory and gets OOM killed on smaller
systems (eg android devices).  Most of the allocation is for "busy work"
to keep the render rings busy and for this we can just re-use the same
few buffers over and over. This enables the test to be run on low end
devices.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Tim Gore <tim.gore@intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>