gpu: ipu-v3: only set non-zero AXI ID for IC when PRG is absent

Using non-zero AXI IDs for anything other than the display channels
collides with the PRG AXI snooping, so only do this if there is no
PRG present.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>