drm/i915: Assert all sg are initialised in fake_dma_object for selftests

Double check that we allocated the right amount of scatterlist elements
for our obj->size.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170225181122.4788-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/selftests/i915_gem_gtt.c

@@ -62,12 +62,14 @@ fake_get_pages(struct drm_i915_gem_object *obj)
 	for (sg = pages->sgl; sg; sg = sg_next(sg)) {
 		unsigned long len = min_t(typeof(rem), rem, BIT(31));
 
+		GEM_BUG_ON(!len);
 		sg_set_page(sg, pfn_to_page(PFN_BIAS), len, 0);
 		sg_dma_address(sg) = page_to_phys(sg_page(sg));
 		sg_dma_len(sg) = len;
 
 		rem -= len;
 	}
+	GEM_BUG_ON(rem);
 
 	obj->mm.madv = I915_MADV_DONTNEED;
 	return pages;