drm/i915: correct self-refresh calculation in "everything off" case

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: Eric Anholt <eric@anholt.net>
Tested-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>