drm/tiny: panel-mipi-dbi: Include <linux/of.h>

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

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20241011144632.90434-7-tzimmermann@suse.de