drm/i915: Protect force_bit with gmbus_mutex

Extend the protection of gmbus_mutex around the force_bit
RMW in intel_gmbus_force_bit(), in case someone gets the
idea of calling it from a separate thread while there's
other stuff happening on the same bus.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1457366220-29409-3-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>

drivers/gpu/drm/i915/intel_i2c.c

@@ -718,11 +718,16 @@ void intel_gmbus_set_speed(struct i2c_adapter *adapter, int speed)
 void intel_gmbus_force_bit(struct i2c_adapter *adapter, bool force_bit)
 {
 	struct intel_gmbus *bus = to_intel_gmbus(adapter);
+	struct drm_i915_private *dev_priv = bus->dev_priv;
+
+	mutex_lock(&dev_priv->gmbus_mutex);
 
 	bus->force_bit += force_bit ? 1 : -1;
 	DRM_DEBUG_KMS("%sabling bit-banging on %s. force bit now %d\n",
 		      force_bit ? "en" : "dis", adapter->name,
 		      bus->force_bit);
+
+	mutex_unlock(&dev_priv->gmbus_mutex);
 }
 
 void intel_teardown_gmbus(struct drm_device *dev)