drm/i915: hide unused intel_panel_set_backlight function

Building i915 without backlight support results in a harmless warning
for intel_panel_set_backlight:

drivers/gpu/drm/i915/intel_panel.c:653:13: error: 'intel_panel_set_backlight' defined but not used [-Werror=unused-function]

This moves it into the CONFIG_BACKLIGHT_CLASS_DEVICE section that
its caller is in.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171127151239.1813673-2-arnd@arndb.de