drm/amdgpu/gfx8: set TC_WB_ACTION_EN in RELEASE_MEM packet

This is the recommended setting from the hw team for newer
versions of the firmware.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org