gpu: ipu-v3: image-convert: use swap()

Use swap() instead of implementing it since it makes code cleaner.

Signed-off-by: Salah Triki <salah.triki@gmail.com>
Link: https://lore.kernel.org/r/20210713140521.GA1873885@pc


[p.zabel@pengutronix.de: add "image-convert:" prefix to commit description]
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>