drm/i915: Avoid NULL deref in get_pages() unwind after error.

Fixes:
  http://bugzilla.kernel.org/show_bug.cgi?id=15527
  NULL pointer dereference in i915_gem_object_save_bit_17_swizzle

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<f82b5d2b>] i915_gem_object_save_bit_17_swizzle+0x5b/0xc0 [i915]
Call Trace:
[<f82aea55>] ? i915_gem_object_put_pages+0x125/0x150 [i915]
[<f82aeb71>] ? i915_gem_object_get_pages+0xf1/0x110 [i915]
[<f82b0de8>] ? i915_gem_object_bind_to_gtt+0xb8/0x2a0 [i915]
[<c02db74d>] ? drm_mm_get_block_generic+0x4d/0x180
[<f82b11cd>] ? i915_gem_mmap_gtt_ioctl+0x16d/0x240 [i915]
[<f82ae786>] ? i915_gem_madvise_ioctl+0x86/0x120 [i915]
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Reported-by: maciej.rutecki@gmail.com
Cc: stable@kernel.org
Reviewed-by: NEric Anholt <eric@anholt.net>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_gem.c

@@ -1466,9 +1466,6 @@ i915_gem_object_put_pages(struct drm_gem_object *obj)
 		obj_priv->dirty = 0;
 
 	for (i = 0; i < page_count; i++) {
-		if (obj_priv->pages[i] == NULL)
-			break;
-
 		if (obj_priv->dirty)
 			set_page_dirty(obj_priv->pages[i]);
 
@@ -2251,7 +2248,6 @@ i915_gem_object_get_pages(struct drm_gem_object *obj,
 	struct address_space *mapping;
 	struct inode *inode;
 	struct page *page;
-	int ret;
 
 	if (obj_priv->pages_refcount++ != 0)
 		return 0;
@@ -2274,11 +2270,9 @@ i915_gem_object_get_pages(struct drm_gem_object *obj,
 					   mapping_gfp_mask (mapping) |
 					   __GFP_COLD |
 					   gfpmask);
-		if (IS_ERR(page)) {
-			ret = PTR_ERR(page);
-			i915_gem_object_put_pages(obj);
-			return ret;
-		}
+		if (IS_ERR(page))
+			goto err_pages;
+
 		obj_priv->pages[i] = page;
 	}
 
@@ -2286,6 +2280,15 @@ i915_gem_object_get_pages(struct drm_gem_object *obj,
 		i915_gem_object_do_bit_17_swizzle(obj);
 
 	return 0;
+
+err_pages:
+	while (i--)
+		page_cache_release(obj_priv->pages[i]);
+
+	drm_free_large(obj_priv->pages);
+	obj_priv->pages = NULL;
+	obj_priv->pages_refcount--;
+	return PTR_ERR(page);
 }
 
 static void sandybridge_write_fence_reg(struct drm_i915_fence_reg *reg)