test/amdgpu: Add emit mem sync flag for test IB

In syncobj test, 3 threads will be created. Sometimes
the first gfx IB and the third sdma IB will use same
physical page. There will be risk that sdma engine will
read gfx IB in the same physical page. So better to flush
the cache before commit the sdma IB.

Signed-off-by: Jinzhou Su <Jinzhou.Su@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>