drm/lima: enable runtime pm

Enable runtime pm by default so GPU suspend when idle
for 200ms. This value can be changed by
autosuspend_delay_ms in device's power sysfs dir.

On Allwinner H3 lima_device_resume takes ~40us and
lima_device_suspend takes ~20us.

Tested-by: Bhushan Shah <bshah@kde.org>
Reviewed-by: Vasily Khoruzhick <anarsoul@gmail.com>
Signed-off-by: Qiang Yu <yuq825@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200421133551.31481-11-yuq825@gmail.com