staging: ftbft: Use backlight helper

staging: ftbft: Use backlight helper

backlight_properties.fb_blank is deprecated. The states it represents
are handled by other properties; but instead of accessing those
properties directly, drivers should use the helpers provided by
backlight.h.

Instead of manually checking the power state in struct
backlight_properties, use backlight_is_blank().

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Javier Martinez Canillas <javierm@redhat.com>
Cc: Len Baker <len.baker@gmx.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-fbdev@vger.kernel.org
Cc: linux-staging@lists.linux.dev
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Stephen Kitt <steve@sk2.org>
Link: https://lore.kernel.org/r/20220607185516.1129900-1-steve@sk2.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>