gpu: ipu-v3: prg: remove counter load enable

The counter load enable bit has no effect when the shadow register
set is activated. As we always operate the PRG with shadow enabled
it is safe to remove this.

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