gpu: ipu-v3: pre: only use internal clock gating

By setting the SFTRST bit, the PRE will be held in the lowest power state
with clocks to the internal blocks gated. When external clock gating is
used (from the external clock controller, or by setting the CLKGATE bit)
the PRE will sporadically fail to start.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Fixes: d2a34232 ("gpu: ipu-v3: add driver for Prefetch Resolve Engine")
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>