drm/i915/ringbuffer: Use the HEAD auto-reporting mechanism

My Sandybridge only reports 0 for the ring buffer registers, causing it
to hang as soon as we exhaust the available ring. As a workaround, take
advantage of our huge ring buffers and use the auto-reporting mechanism
to update the status page with the HEAD location every 64 KiB.

Cherry-picked from 6aa56062.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=31404


Tested-by: Zhao Jian <jian.j.zhao@intel.com>
Cc: stable@kernel.org
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>