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

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>