test/amdgpu: remove static varible in Syncobj test

In syncobj test, wait thread and signal thread create
simultaneously. The ptr for GFX IB and SDMA IP should be
operated separately. With static, there will be risk that
GFX NOP is in SDMA IB or SDMA NOP is in GFX IB, then GFX or
SDMA hang caused.

Signed-off-by: Jinzhou Su <Jinzhou.Su@amd.com>
Signed-off-by: Marek Olšák <marek.olsak@amd.com>