1, for sriov, we need 8dw for the gfx fence due to CP
behaviour
2, cleanup wrong logic in wptr/rptr wb alloc and free

Change-Id: Ifbfed17a4621dae57244942ffac7de1743de0294
Signed-off-by: NMonk Liu <Monk.Liu@amd.com>
Signed-off-by: NXiangliang Yu <Xiangliang.Yu@amd.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Reviewed-by: NChristian König <christian.koenig@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>