drm/amdgpu/gfx9: cache DB_DEBUG2 and make it available to userspace

Userspace needs to query this value to work around a hw bug in
certain cases.

Acked-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>