media: hantro: h264: Move reference picture number to a helper

Add a hantro_h264_get_ref_nbr() helper function to get the reference
picture numbers. This will be used by the Rockchip VDPU2 H.264 driver.

This idea was originally proposed by Jonas Karlman in
"[RFC 09/12] media: hantro: Refactor G1 H264 code"
posted a while ago.

Link: https://lore.kernel.org/linux-media/HE1PR06MB401165F2BA0AD8A634FDFAF2ACBF0@HE1PR06MB4011.eurprd06.prod.outlook.com/



Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Tested-by: Alex Bee <knaerzche@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>