drm/i915/selftests: Fix error path for ggtt walk_hole()

The patch 6e32ab3d4777: "drm/i915: Fill different pages of the GTT"
from Feb 13, 2017, leads to the following static checker warning:

        drivers/gpu/drm/i915/selftests/i915_gem_gtt.c:583 walk_hole()
        error: 'vma' dereferencing possible ERR_PTR()

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 6e32ab3d4777 ("drm/i915: Fill different pages of the GTT"
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: <drm-intel-fixes@lists.freedesktop.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170313100750.2685-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>

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

@@ -539,7 +539,7 @@ static int walk_hole(struct drm_i915_private *i915,
 		vma = i915_vma_instance(obj, vm, NULL);
 		if (IS_ERR(vma)) {
 			err = PTR_ERR(vma);
-			goto err;
+			goto err_put;
 		}
 
 		for (addr = hole_start;
@@ -550,7 +550,7 @@ static int walk_hole(struct drm_i915_private *i915,
 				pr_err("%s bind failed at %llx + %llx [hole %llx- %llx] with err=%d\n",
 				       __func__, addr, vma->size,
 				       hole_start, hole_end, err);
-				goto err;
+				goto err_close;
 			}
 			i915_vma_unpin(vma);
 
@@ -559,14 +559,14 @@ static int walk_hole(struct drm_i915_private *i915,
 				pr_err("%s incorrect at %llx + %llx\n",
 				       __func__, addr, vma->size);
 				err = -EINVAL;
-				goto err;
+				goto err_close;
 			}
 
 			err = i915_vma_unbind(vma);
 			if (err) {
 				pr_err("%s unbind failed at %llx + %llx  with err=%d\n",
 				       __func__, addr, vma->size, err);
-				goto err;
+				goto err_close;
 			}
 
 			GEM_BUG_ON(drm_mm_node_allocated(&vma->node));
@@ -575,13 +575,14 @@ static int walk_hole(struct drm_i915_private *i915,
 					"%s timed out at %llx\n",
 					__func__, addr)) {
 				err = -EINTR;
-				goto err;
+				goto err_close;
 			}
 		}
 
-err:
+err_close:
 		if (!i915_vma_is_ggtt(vma))
 			i915_vma_close(vma);
+err_put:
 		i915_gem_object_put(obj);
 		if (err)
 			return err;