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 <[e-mail]>
Cc: "Noralf Trønnes" <[e-mail]>
Reviewed-by: Noralf Trønnes <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/tiny/panel-mipi-dbi.c

···
 #include <linux/firmware.h>
 #include <linux/gpio/consumer.h>
 #include <linux/module.h>
+#include <linux/of.h>
 #include <linux/property.h>
 #include <linux/regulator/consumer.h>
 #include <linux/spi/spi.h>