media: hantro: imx: reuse MB_DIM define

Swap the hardcoded 16 with MB_DIM define.

Fixes: 8e4aaa68 ("media: hantro: add initial i.MX8MQ support")
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Reviewed-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>