drm/amdgpu/gfx9: set additional bits on CP halt

Need to set the pipe reset and cache invalidation bits
on halt otherwise we can get stale state if the CP firmware
changes (e.g., on module unload and reload).

Reviewed-by: Srinivasan Shanmugam <srinivasan.shanmugam@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>