drm/i915: fix lookup_power_well for power wells without any domain

The current lookup code wouldn't find a power well if it's not in any
power domain. There wasn't any power wells before but an upcoming patch
will detach the power domains from power well#1 and the MISC IO power
wells, so fix things up accordingly.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Patrik Jakobsson <patrik.jakobsson@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1446657859-9598-3-git-send-email-imre.deak@intel.com