drm/i915: Avoid dereference past end of page array in gen8_ppgtt_insert_entries()

The bug from gen6_ppgtt_insert_entries() was replicated into
gen8_ppgtt_insert_entries(). This applies the fix for the OOPS from the
previous patch to the gen8 routine.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Ben Widawsky <benjamin.widawsky@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -297,23 +297,23 @@ static void gen8_ppgtt_insert_entries(struct i915_address_space *vm,
 	unsigned act_pte = first_entry % GEN8_PTES_PER_PAGE;
 	struct sg_page_iter sg_iter;
 
-	pt_vaddr = kmap_atomic(&ppgtt->gen8_pt_pages[act_pt]);
+	pt_vaddr = NULL;
 	for_each_sg_page(pages->sgl, &sg_iter, pages->nents, 0) {
-		dma_addr_t page_addr;
+		if (pt_vaddr == NULL)
+			pt_vaddr = kmap_atomic(&ppgtt->gen8_pt_pages[act_pt]);
 
-		page_addr = sg_dma_address(sg_iter.sg) +
-				(sg_iter.sg_pgoffset << PAGE_SHIFT);
-		pt_vaddr[act_pte] = gen8_pte_encode(page_addr, cache_level,
-						    true);
+		pt_vaddr[act_pte] =
+			gen8_pte_encode(sg_page_iter_dma_address(&sg_iter),
+					cache_level, true);
 		if (++act_pte == GEN8_PTES_PER_PAGE) {
 			kunmap_atomic(pt_vaddr);
+			pt_vaddr = NULL;
 			act_pt++;
-			pt_vaddr = kmap_atomic(&ppgtt->gen8_pt_pages[act_pt]);
 			act_pte = 0;
-
 		}
 	}
-	kunmap_atomic(pt_vaddr);
+	if (pt_vaddr)
+		kunmap_atomic(pt_vaddr);
 }
 
 static void gen8_ppgtt_cleanup(struct i915_address_space *vm)