drm/i915: i915_pages_create_for_stolen should return err ptr

drm/i915: i915_pages_create_for_stolen should return err ptr

When gathering the pages from our backing storage we expect get_pages()
to either give us our sg_table or an err ptr. However when gathering our
fake pages for stolen memory we may return NULL in the event of a
failure. To prevent any funny business we should therefore return the
proper err ptr value.

Fixes: 03ac84f1830e ("drm/i915: Pass around sg_table to get_pages/put_pages backend")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1479488536-6168-1-git-send-email-matthew.auld@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>