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>