drm/i915: Protect debugfs per_file_stats with RCU lock

If we make sure we grab a strong reference to each object as we dump it,
we can reduce the locks outside of our iterators to an rcu_read_lock.

This should prevent errors like:
[ 2138.371911] BUG: KASAN: use-after-free in per_file_stats+0x43/0x380 [i915]
[ 2138.371924] Read of size 8 at addr ffff888223651000 by task cat/8293

[ 2138.371947] CPU: 0 PID: 8293 Comm: cat Not tainted 5.3.0-rc6-CI-Custom_4352+ #1
[ 2138.371953] Hardware name: To Be Filled By O.E.M. To Be Filled By O.E.M./J4205-ITX, BIOS P1.40 07/14/2017
[ 2138.371959] Call Trace:
[ 2138.371974]  dump_stack+0x7c/0xbb
[ 2138.372099]  ? per_file_stats+0x43/0x380 [i915]
[ 2138.372108]  print_address_description+0x73/0x3a0
[ 2138.372231]  ? per_file_stats+0x43/0x380 [i915]
[ 2138.372352]  ? per_file_stats+0x43/0x380 [i915]
[ 2138.372362]  __kasan_report+0x14e/0x192
[ 2138.372489]  ? per_file_stats+0x43/0x380 [i915]
[ 2138.372502]  kasan_report+0xe/0x20
[ 2138.372625]  per_file_stats+0x43/0x380 [i915]
[ 2138.372751]  ? i915_panel_show+0x110/0x110 [i915]
[ 2138.372761]  idr_for_each+0xa7/0x160
[ 2138.372773]  ? idr_get_next_ul+0x110/0x110
[ 2138.372782]  ? do_raw_spin_lock+0x10a/0x1d0
[ 2138.372923]  print_context_stats+0x264/0x510 [i915]

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: David Weinehall <david.weinehall@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190903062133.27360-1-chris@chris-wilson.co.uk