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>

drivers/gpu/drm/i915/intel_runtime_pm.c

@@ -856,7 +856,7 @@ static void bxt_dpio_cmn_power_well_enable(struct drm_i915_private *dev_priv,
 					   struct i915_power_well *power_well)
 {
 	enum skl_disp_power_wells power_well_id = power_well->data;
-	struct i915_power_well *cmn_a_well;
+	struct i915_power_well *cmn_a_well = NULL;
 
 	if (power_well_id == BXT_DPIO_CMN_BC) {
 		/*
@@ -869,7 +869,7 @@ static void bxt_dpio_cmn_power_well_enable(struct drm_i915_private *dev_priv,
 
 	bxt_ddi_phy_init(dev_priv, bxt_power_well_to_phy(power_well));
 
-	if (power_well_id == BXT_DPIO_CMN_BC)
+	if (cmn_a_well)
 		intel_power_well_put(dev_priv, cmn_a_well);
 }