media: renesas: Set min_buffers_needed to 16

vb2 queue_setup checks for a minimum number of buffers so set
min_buffers_needed to 16 and remove the useless check in
rcar_drif_queue_setup().

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
CC: Philipp Zabel <p.zabel@pengutronix.de>