drm/i915: Add an rcu_barrier option to i915_drop_caches

Sometimes a test has to wait for RCU to complete a grace period and
perform its callbacks, for example waiting for a close(fd) to actually
perform the fput(filp) and so trigger all the callbacks such as closing
GEM contexts. There is no trivial means of triggering an RCU barrier
from userspace, so add one for our convenience in
debugfs/i915_drop_caches

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191011173823.20432-1-chris@chris-wilson.co.uk