etnaviv: perfmon: fix total and idle HI cyleces readout

As seen at CodeAurora's linux-imx git repo in imx_4.19.35_1.0.0 branch.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>