drm/i915: don't pwrite tiled objects through the gtt

... we will botch up the bit17 swizzling. Furthermore tiled pwrite is
a (now) unused slowpath, so no one really cares.

This fixes the last swizzling issues I have with i-g-t on my bit17
swizzling i915G. No regression, it's been broken since the dawn of
gem, but it's nice for regression tracking when really _all_ i-g-t
tests work.

Actually this is not true, Chris Wilson noticed while reviewing this
patch that the commit

commit d9e86c0e
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Wed Nov 10 16:40:20 2010 +0000

    drm/i915: Pipelined fencing [infrastructure]

contained a functional change that broke things.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-Off-by: Daniel Vetter <daniel.vetter@ffwll.ch>