drm/i915: Hold struct mutex whilst pinning power context bo.

Hugh found an error path where we were attempting to unref a bo without
holding the struct mutex:

  [drm:intel_init_clock_gating] *ERROR* failed to pin power context: -16
  ------------[ cut here ]------------
  WARNING: at drivers/gpu/drm/drm_gem.c:438 drm_gem_object_free+0x20/0x5e()
  Hardware name: ESPRIMO Mobile V5505
  Modules linked in: snd_pcm_oss snd_mixer_oss snd_seq snd_seq_device
  Pid: 3793, comm: s2ram Not tainted 2.6.33-rc2 #4
  Call Trace:
   [<7815298e>] warn_slowpath_common+0x59/0x6b
   [<781529b3>] warn_slowpath_null+0x13/0x18
   [<78317c1a>] ? drm_gem_object_free+0x20/0x5e
   [<78317c1a>] drm_gem_object_free+0x20/0x5e
   [<78317bfa>] ? drm_gem_object_free+0x0/0x5e
   [<7829df11>] kref_put+0x38/0x45
   [<7833a5f0>] intel_init_clock_gating+0x232/0x271
   [<78317bfa>] ? drm_gem_object_free+0x0/0x5e
   [<7832c307>] i915_restore_state+0x21a/0x2b3
   [<7832379d>] i915_resume+0x3c/0xbb
   [<78174fe5>] ? trace_hardirqs_on_caller+0xfc/0x123
   [<7831c756>] ? drm_class_resume+0x0/0x3e
   [<7831c78d>] drm_class_resume+0x37/0x3e
   [<78351e0a>] legacy_resume+0x1e/0x51
   [<78351ece>] device_resume+0x91/0xab
   [<7831c756>] ? drm_class_resume+0x0/0x3e
   [<78352226>] dpm_resume+0x58/0x10f
   [<783522fb>] dpm_resume_end+0x1e/0x2c
   [<78180f80>] suspend_devices_and_enter+0x61/0x84
   [<78180ff8>] enter_state+0x55/0x83
   [<7818091c>] state_store+0x94/0xaa
   [<7829d09e>] kobj_attr_store+0x1e/0x23
   [<782098e0>] sysfs_write_file+0x66/0x99
   [<781cd2f0>] vfs_write+0x8a/0x108
   [<781cd408>] sys_write+0x3c/0x63
   [<78125c10>] sysenter_do_call+0x12/0x36
  ---[ end trace a343537f29950fda ]---

It is in fact slightly more insiduous that first appears since we are
attempting to not just free the object without the lock, but are trying
to do the whole bo manipulation without holding the lock.

Reported-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org
Signed-off-by: Eric Anholt <eric@anholt.net>