drm/imx: make static read-only array channel_offsets const

The static array channel_offsets is read-only so it make sense to make
it const.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://lore.kernel.org/r/20220123223417.6244-1-colin.i.king@gmail.com