drm/amdgpu: Enable gfx cache probing on HDP write for arcturus

This allows gfx cache to be probed and invalidated (for none-dirty cache lines)
on a HDP write (from either another GPU or CPU). This should work only for the
memory mapped as RW memory type newly added for arcturus, to achieve some cache
coherence b/t multiple memory clients.

Signed-off-by: Oak Zeng <Oak.Zeng@amd.com>
Acked-by: Christian Konig <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>