drm/msm: Disallow relocs on a6xx+

Mesa stopped using these pretty early in a6xx bringup[1].  Take advantage
of this to disallow some legacy UABI.

[1] https://gitlab.freedesktop.org/mesa/mesa/-/commit/7ef722861b691ce99be3827ed05f8c0ddf2cd66e



Signed-off-by: Rob Clark <robdclark@chromium.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Patchwork: https://patchwork.freedesktop.org/patch/551175/