drm/etnaviv: Drop the <linux/pm_runtime.h> header

Currently, the etnaviv_gem_submit.c isn't call any runtime power management
functions. So drop this unused header, we can include it back when it
really get used though.

Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>
Signed-off-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>