drm/i915: move lookup_power_well() up

There's no need for that forward declaration.

Cc: Imre Deak <imre.deak@intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180820233139.11936-4-paulo.r.zanoni@intel.com