When we return pages to the system, we release control over them and
should defensively return them to the CPU write domain so that we catch
any external writes on reacquiring them (e.g. to transparently
swapout/swapin). While we did this defensive clflushing for ordinary
shmem pages, it was forgotten for userptr. Fortunately, userptr objects
are normally cache coherent and so oblivious to the forgotten domain
tracking.

References: a679f58d ("drm/i915: Flush pages on acquisition")
References: 754a2544 ("drm/i915: Skip object locking around a no-op set-domain ioctl")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190331094620.15185-1-chris@chris-wilson.co.uk