drm/i915: Reset last_retired_head when resetting ring

When we reset the ring control registers, including the HEAD and TAIL of
the ring, we also need to reset associated state. In this instance, we
were failing to reset the cached value of ring->last_retired_head and so
upon the first request for more space following a resume would
potentially (depending on a narrow race window) believe that the HEAD had
advanced much further than reality.

This is a regression from:

commit a71d8d94
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Feb 15 11:25:36 2012 +0000

    drm/i915: Record the tail at each request and use it to estimate the head

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org # 3.4
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>