Otherwise we get flooded by the kernel warning us that we are doing
long sequences of IO without serialisation. For example,

 WARNING: CPU: 0 PID: 11136 at drivers/gpu/drm/i915/intel_sideband.c:40 vlv_sideband_rw+0x48/0x1ef()
 Modules linked in:
 CPU: 0 PID: 11136 Comm: kworker/u2:0 Tainted: G        W    3.11.0-rc2+ #4
 Call Trace:
  [<c2028564>] ?  warn_slowpath_common+0x63/0x78
  [<c227ad43>] ?  vlv_sideband_rw+0x48/0x1ef
  [<c20285dd>] ?  warn_slowpath_null+0xf/0x13
  [<c227ad43>] ?  vlv_sideband_rw+0x48/0x1ef
  [<c227b060>] ?  vlv_dpio_write+0x1c/0x21
  [<c2262b3b>] ?  intel_dp_set_signal_levels+0x24a/0x385
  [<c2264909>] ?  intel_dp_complete_link_train+0x25/0x1d1
  [<c2264c55>] ?  intel_dp_check_link_status+0xf7/0x106
  [<c2238ced>] ?  i915_hotplug_work_func+0x17b/0x221
  [<c203a204>] ?  process_one_work+0x12e/0x210
  [<c203a5e4>] ?  worker_thread+0x116/0x1ad
  [<c203a4ce>] ?  rescuer_thread+0x1cb/0x1cb
  [<c203d8f5>] ?  kthread+0x67/0x6c
  [<c2457ebb>] ?  ret_from_kernel_thread+0x1b/0x30
  [<c203d88e>] ?  init_completion+0x18/0x18

v2: Retire the locking in vlv_crtc_enable() and do it close to the meat.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
[danvet: Squash in a s/mutex_lock/mutex_unlock/ fixup spotted by the 0
day kernel build/coccinelle and reported by Dan Carpenter.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>