gpu: ipu-v3: Allow channel burst locking on i.MX6 only

The IDMAC_LOCK_EN registers on i.MX51 have a different layout, and on
i.MX53 enabling the lock feature causes bursts to get lost. Restrict
enabling the burst lock feature to i.MX6.

Reported-by: Patrick Brünn <P.Bruenn@beckhoff.com>
Fixes: 790cb4c7 ("drm/imx: lock scanout transfers for consecutive bursts")
Tested-by: Patrick Brünn <P.Bruenn@beckhoff.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>