drm/i915: Add struct_mutex locking for debugs/i915_gem_framebuffer

drm/i915: Add struct_mutex locking for debugs/i915_gem_framebuffer

Since describe_obj() looks at state guarded by the struct_mutex, we need
to be holding it.

[  580.201054] drv_suspend: starting subtest debugfs-reader
[  580.239652] ------------[ cut here ]------------
[  580.239696] WARNING: CPU: 0 PID: 920 at include/linux/list_check.h:25 describe_obj+0x419/0x440()
[  580.239725] CPU: 0 PID: 920 Comm: cat Not tainted 4.5.0-rc6+ #835
[  580.239745] Hardware name:                  /NUC5CPYB, BIOS PYBSWCEL.86A.0027.2015.0507.1758 05/07/2015
[  580.239767]  0000000000000000 ffff88027554fcf8 ffffffff812c1135 0000000000000000
[  580.239815]  ffffffff8193dc42 ffff88027554fd30 ffffffff8107419d ffff880071727c00
[  580.239858]  ffff8802757d8000 ffffffff818f693c ffffffff818f693c ffff8802757b9048
[  580.239896] Call Trace:
[  580.239917]  [<ffffffff812c1135>] dump_stack+0x67/0x92
[  580.239939]  [<ffffffff8107419d>] warn_slowpath_common+0x7d/0xb0
[  580.239959]  [<ffffffff810742ba>] warn_slowpath_null+0x1a/0x20
[  580.239981]  [<ffffffff813ce579>] describe_obj+0x419/0x440
[  580.240006]  [<ffffffff813ced22>] i915_gem_framebuffer_info+0xa2/0x100
[  580.240033]  [<ffffffff811a9286>] seq_read+0xe6/0x3b0
[  580.240059]  [<ffffffff81182288>] __vfs_read+0x28/0xd0
[  580.240085]  [<ffffffff81173378>] ? SyS_fadvise64+0x228/0x2c0
[  580.240112]  [<ffffffff811823b2>] vfs_read+0x82/0x110
[  580.240137]  [<ffffffff811827d9>] SyS_read+0x49/0xa0
[  580.240162]  [<ffffffff815bac57>] entry_SYSCALL_64_fastpath+0x12/0x6b
[  580.240187] ---[ end trace 3e2cbf34576c9878 ]---
[  580.281900] ------------[ cut here ]------------

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1459689261-7920-1-git-send-email-chris@chris-wilson.co.uk