If no planes are enabled, the self-refresh calculation may end up doing
a divide by zero.  This patch should prevent that by making sure at
least one of the CRTCs had a valid hdisplay value.
Reported-by: NEric Anholt <eric@anholt.net>
Tested-by: NEric Anholt <eric@anholt.net>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NEric Anholt <eric@anholt.net>