drm/i915: Fix cap check for intel_dp_aux_backlight driver

intel_dp_aux_backlight driver should check for the
DP_EDP_BACKLIGHT_BRIGHTNESS_AUX_SET_CAP before enable the driver.

Signed-off-by: Puthikorn Voravootivat <puthik@chromium.org>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170511230225.142870-2-puthik@chromium.org