Since tweaking i915_vma_compare() we allowed constructors to skip
clearing the ggtt_view believing that we didn't access the unused
members. That, as it turns out, was not entirely true. In particular,
i915_gem_fault() uses

    ret = remap_io_mapping(area,
	    area->vm_start + (vma->ggtt_view.partial.offset << PAGE_SHIFT),
	    (ggtt->mappable_base + vma->node.start) >> PAGE_SHIFT,
	    min_t(u64, vma->size, area->vm_end - area->vm_start),
	    &ggtt->mappable);

i.e. the ggtt_view.partial for both normal and partial views. If we
allowed garbage into the normal vma->ggtt_view and then try userspace
tried to mmap it, we could explode in an unobvious fashion.

Fixes: 7b92c047 ("drm/i915: Eliminate superfluous i915_ggtt_view_rotated")
Fixes: 3bf4d575 ("drm/i915: Stop clearing i915_ggtt_view")
Reported-by: Matthew Auld <matthew.william.auld@gmail.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170123145245.3972-1-chris@chris-wilson.co.ukTested-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
(cherry picked from commit 7c518460)