drm/amdgpu: add optional ring to *_hdp callbacks

This adds an optional ring to the invalidate_hdp and flush_hdp
callbacks. If the ring isn't specified or the emit_wreg function not
available the HDP operation will be done with the CPU otherwise by
writing on the ring.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Chunming Zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>