drm/i915: fix intel_backlight_device_register declaration

The alternative intel_backlight_device_register() definition apparently
never got used, but I have now run into a case of i915 being compiled
without CONFIG_BACKLIGHT_CLASS_DEVICE, resulting in a number of
identical warnings:

drivers/gpu/drm/i915/intel_drv.h:1739:12: error: 'intel_backlight_device_register' defined but not used [-Werror=unused-function]

This marks the function as 'inline', which was surely the original
intention here.

Fixes: 1ebaa0b9 ("drm/i915: Move backlight registration to connector registration")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171127151239.1813673-1-arnd@arndb.de