efi/bgrt: Drop __initdata from bgrt_image_size

bgrt_image_size is necessary to (optionally) show the boot graphics from
the efifb code. The efifb driver is a platform driver, using a normal
driver probe() driver callback. So even though it is always builtin it
cannot reference __initdata.

Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>