amd/amdgpu: remove test ib on hw ring

test ib function is not necessary on hw ring,
so remove it.

v2: squash in NULL check fix

Signed-off-by: JesseZhang <Jesse.Zhang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>