drm/i915: kill pointless clearing of dev_priv->hws_map

We kzalloc dev_priv, and we never use hws_map in intel_ringbuffer.c.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>