drm/amdgpu:fix gfx fence allocate size

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: Monk Liu <Monk.Liu@amd.com>
Signed-off-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>