drm/lima: always set page directory when switch vm

We need to flush TLB anyway before every task start, and the
page directory will be set to empty vm after suspend/resume,
so always set it to the task vm even no ctx switch happens.

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/20200421133551.31481-5-yuq825@gmail.com