drm: self_refresh: Fix a reversed condition in drm_self_refresh_helper_cleanup()

This test is flipped around so it either leads to a memory leak or a
NULL dereference.

Fixes: 1452c25b ("drm: Add helpers to kick off self refresh mode in drivers")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190619100141.GA28596@mwanda