drm/i915: Fix nesting of filelist_mutex vs struct_mutex in i915_ppgtt_info

An unlikely ABBA deadlock in debugfs that no one has reported.

[  284.922349] ======================================================
[  284.922355] [ INFO: possible circular locking dependency detected ]
[  284.922361] 4.8.0-rc2+ #430 Tainted: G        W
[  284.922366] -------------------------------------------------------
[  284.922371] cat/1197 is trying to acquire lock:
[  284.922376]  (&dev->filelist_mutex){+.+...}, at: [<ffffffffa0055ba2>] i915_ppgtt_info+0x82/0x390 [i915]
[  284.922423]
[  284.922423] but task is already holding lock:
[  284.922429]  (&dev->struct_mutex){+.+.+.}, at: [<ffffffffa0055b55>] i915_ppgtt_info+0x35/0x390 [i915]
[  284.922465]
[  284.922465] which lock already depends on the new lock.
[  284.922465]
[  284.922471]
[  284.922471] the existing dependency chain (in reverse order) is:
[  284.922477]
-> #1 (&dev->struct_mutex){+.+.+.}:
[  284.922493]        [<ffffffff81087710>] lock_acquire+0x60/0x80
[  284.922505]        [<ffffffff8143e96f>] mutex_lock_nested+0x5f/0x360
[  284.922520]        [<ffffffffa004f877>] print_context_stats+0x37/0xf0 [i915]
[  284.922549]        [<ffffffffa00535f5>] i915_gem_object_info+0x265/0x490 [i915]
[  284.922581]        [<ffffffff81144491>] seq_read+0xe1/0x3b0
[  284.922592]        [<ffffffff811f77b3>] full_proxy_read+0x83/0xb0
[  284.922604]        [<ffffffff8111ba03>] __vfs_read+0x23/0x110
[  284.922616]        [<ffffffff8111c9b9>] vfs_read+0x89/0x110
[  284.922626]        [<ffffffff8111dbf4>] SyS_read+0x44/0xa0
[  284.922636]        [<ffffffff81442be9>] entry_SYSCALL_64_fastpath+0x1c/0xac
[  284.922648]
-> #0 (&dev->filelist_mutex){+.+...}:
[  284.922667]        [<ffffffff810871fc>] __lock_acquire+0x10fc/0x1270
[  284.922678]        [<ffffffff81087710>] lock_acquire+0x60/0x80
[  284.922689]        [<ffffffff8143e96f>] mutex_lock_nested+0x5f/0x360
[  284.922701]        [<ffffffffa0055ba2>] i915_ppgtt_info+0x82/0x390 [i915]
[  284.922729]        [<ffffffff81144491>] seq_read+0xe1/0x3b0
[  284.922739]        [<ffffffff811f77b3>] full_proxy_read+0x83/0xb0
[  284.922750]        [<ffffffff8111ba03>] __vfs_read+0x23/0x110
[  284.922761]        [<ffffffff8111c9b9>] vfs_read+0x89/0x110
[  284.922771]        [<ffffffff8111dbf4>] SyS_read+0x44/0xa0
[  284.922781]        [<ffffffff81442be9>] entry_SYSCALL_64_fastpath+0x1c/0xac
[  284.922793]
[  284.922793] other info that might help us debug this:
[  284.922793]
[  284.922809]  Possible unsafe locking scenario:
[  284.922809]
[  284.922818]        CPU0                    CPU1
[  284.922825]        ----                    ----
[  284.922831]   lock(&dev->struct_mutex);
[  284.922842]                                lock(&dev->filelist_mutex);
[  284.922854]                                lock(&dev->struct_mutex);
[  284.922865]   lock(&dev->filelist_mutex);
[  284.922875]
[  284.922875]  *** DEADLOCK ***
[  284.922875]
[  284.922888] 3 locks held by cat/1197:
[  284.922895]  #0:  (debugfs_srcu){......}, at: [<ffffffff811f7730>] full_proxy_read+0x0/0xb0
[  284.922919]  #1:  (&p->lock){+.+.+.}, at: [<ffffffff811443e8>] seq_read+0x38/0x3b0
[  284.922942]  #2:  (&dev->struct_mutex){+.+.+.}, at: [<ffffffffa0055b55>] i915_ppgtt_info+0x35/0x390 [i915]
[  284.922983]

Fixes: 1d2ac403 ("drm: Protect dev->filelist with its own mutex")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160822132820.21725-1-chris@chris-wilson.co.uk