gpu: ipu-v3: Fix IRT usage

There can be multiple IC tasks using the IRT, so the IRT needs
a separate use counter. Create a private ipu_irt_enable() to
enable the IRT module when any IC task requires rotation, and
ipu_irt_disable() when a task no longer needs the IRT.

Signed-off-by: Steve Longerbeam <steve_longerbeam@mentor.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>