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 d9e86c0
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>

drivers/gpu/drm/i915/i915_gem.c

@@ -876,6 +876,7 @@ i915_gem_pwrite_ioctl(struct drm_device *dev, void *data,
 
 	if (obj->gtt_space &&
 	    obj->cache_level == I915_CACHE_NONE &&
+	    obj->tiling_mode == I915_TILING_NONE &&
 	    obj->map_and_fenceable &&
 	    obj->base.write_domain != I915_GEM_DOMAIN_CPU) {
 		ret = i915_gem_gtt_pwrite_fast(dev, obj, args, file);