drm/via: attempt again to stabilise the AGP DMA command submission.

It's worth remembering that all new bright ideas on how to make this command reader work properly and according to docs will probably fail :( Bring in some old code.

Also allow a larger SG-DMA download stride, and remove unnecessary waits for
command regulators pauses.

Signed-off-by: Dave Airlie <airlied@redhat.com>