drm/panel: panel-sony-acx565akm: Use backlight power constants

Replace FB_BLANK_ constants with their counterparts from the
backlight subsystem. The values are identical, so there's no
change in functionality or semantics.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Neil Armstrong <neil.armstrong@linaro.org>
Cc: Jessica Zhang <quic_jesszhan@quicinc.com>
Reviewed-by: Jessica Zhang <quic_jesszhan@quicinc.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240731122311.1143153-8-tzimmermann@suse.de