drm/panel: Include <linux/of.h>

Include <linux/of.h> directly to get of_device_is_available(). Avoids
the proxy include via <linux/backlight.h>

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Neil Armstrong <neil.armstrong@linaro.org>
Cc: Jessica Zhang <quic_jesszhan@quicinc.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20241011144632.90434-3-tzimmermann@suse.de