drm/i915: Do not allow buffers at offset 0

This is primarily a band aid for an unexplainable error in
gem_reloc_vs_gpu/forked-faulting-reloc-thrashing. Essentially as soon as
a relocated buffer (which had a non-zero presumed offset) moved to
offset 0, something goes bad. Since I have been unable to solve this,
and potentially this is a good thing to do anyway, since many things can
accidentally write to offset 0, why not?

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -3280,9 +3280,11 @@ i915_gem_object_bind_to_vm(struct drm_i915_gem_object *obj,
 	WARN_ON(!list_is_singular(&obj->vma_list));
 
 search_free:
+	/* FIXME: Some tests are failing when they receive a reloc of 0. To
+	 * prevent this, we simply don't allow the 0th offset. */
 	ret = drm_mm_insert_node_in_range_generic(&vm->mm, &vma->node,
 						  size, alignment,
-						  obj->cache_level, 0, gtt_max,
+						  obj->cache_level, 1, gtt_max,
 						  DRM_MM_SEARCH_DEFAULT);
 	if (ret) {
 		ret = i915_gem_evict_something(dev, vm, size, alignment,