drm/amdgpu: Add new ring callback to insert memory sync

Used to flush and invalidate various caches.

v2: Rename function hook

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>