drm/i915: Silence GCC warning for cmn_a_well

drm/i915: Silence GCC warning for cmn_a_well

Just make the logic simple enough for even GCC to understand (and
foolproof against random changes):

drivers/gpu/drm/i915/intel_runtime_pm.c: warning: 'cmn_a_well' may be
used uninitialized in this function [-Wuninitialized]:  => 871:23

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1471284383-22324-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Imre Deak <imre.deak@intel.com>