drm/i915: use for_each_power_well in lookup_power_well()

Use the nice helper function to make the implementation simpler.

v2: Rebase.

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com> (v1)
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180820233139.11936-3-paulo.r.zanoni@intel.com