drm/i915: Only wait one vblank when disabling crc if the pipe is on

Otherwise we incur an unsightly WARNING. The mutex locking is a bit
overkill, but it curbs races and eventially we might grow a locking
check in the vblank wait code to make sure the right crtc lock is
held.

This is fallout from

commit 9393707190194eb8b42e412b444a03331db6862f
Author:     Jesse Barnes <jbarnes@virtuousgeek.org>
AuthorDate: Fri Apr 4 16:12:09 2014 -0700

    drm/i915: warn when a vblank wait times out

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=79612
Tested-by: Guo Jinxian <jinxianx.guo@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: Rebase on top of drm core ww locking changes.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_debugfs.c

@@ -2923,11 +2923,16 @@ static int pipe_crc_set_source(struct drm_device *dev, enum pipe pipe,
 	/* real source -> none transition */
 	if (source == INTEL_PIPE_CRC_SOURCE_NONE) {
 		struct intel_pipe_crc_entry *entries;
+		struct intel_crtc *crtc =
+			to_intel_crtc(dev_priv->pipe_to_crtc_mapping[pipe]);
 
 		DRM_DEBUG_DRIVER("stopping CRCs for pipe %c\n",
 				 pipe_name(pipe));
 
-		intel_wait_for_vblank(dev, pipe);
+		drm_modeset_lock(&crtc->base.mutex, NULL);
+		if (crtc->active)
+			intel_wait_for_vblank(dev, pipe);
+		drm_modeset_unlock(&crtc->base.mutex);
 
 		spin_lock_irq(&pipe_crc->lock);
 		entries = pipe_crc->entries;