drm/etnaviv: User FOLL_LONGTERM in userptr

There's no mmu notifier or anything like that, releasing this pin is
entirely up to userspace. Hence FOLL_LONGTERM.

No cc: stable for this patch since a lot of the infrastructure around
FOLL_LONGETRM (like not allowing it for pages currently sitting in
ZONE_MOVEABLE before they're migrated) is still being worked on. So
not big benefits yet.

Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Russell King <linux+etnaviv@armlinux.org.uk>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: etnaviv@lists.freedesktop.org
Link: https://patchwork.freedesktop.org/patch/msgid/20210301095254.1946084-2-daniel.vetter@ffwll.ch