drm/i915/dsi: Fix chv_exec_gpio disabling the GPIOs it is setting

Set the CHV_GPIO_GPIOEN bit when updating GPIOs from chv_exec_gpio.

Fixes: a0a6d4ff

 ("drm/i915/dsi: add support for gpio elements on CHV")
Cc: stable@vger.kernel.org
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161201202925.12220-3-hdegoede@redhat.com
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
(cherry picked from commit b2b45fcd

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>