drm: Take mode_config.mutex in setcrtc ioctl

Legacy drivers insist that we really take all the locks in this path,
and the harm in doing so is minimal.

v2: Like git add, it exists :(

Fixes: 2ceb585a956c ("drm: Add explicit acquire ctx handling around ->set_config")
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Cc: Alex Deucher <alexdeucher@gmail.com>
Reported-by: Alex Deucher <alexdeucher@gmail.com>
Acked-and-tested-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170406190654.6733-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_crtc.c

@@ -576,6 +576,7 @@ int drm_mode_setcrtc(struct drm_device *dev, void *data,
 	}
 	DRM_DEBUG_KMS("[CRTC:%d:%s]\n", crtc->base.id, crtc->name);
 
+	mutex_lock(&crtc->dev->mode_config.mutex);
 	drm_modeset_acquire_init(&ctx, 0);
 retry:
 	ret = drm_modeset_lock_all_ctx(crtc->dev, &ctx);
@@ -721,6 +722,7 @@ out:
 	}
 	drm_modeset_drop_locks(&ctx);
 	drm_modeset_acquire_fini(&ctx);
+	mutex_unlock(&crtc->dev->mode_config.mutex);
 
 	return ret;
 }