drm/exynos: drop struct_mutex from exynos_gem_map_sgt_with_dma

The sg table isn't refcounted, there's no corresponding locking for
unmapping and drm_map_sg is ok with being called concurrently.

So drop the locking since it doesn't protect anything.

Cc: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1459330852-27668-8-git-send-email-daniel.vetter@ffwll.ch

drivers/gpu/drm/exynos/exynos_drm_gem.c

@@ -388,16 +388,12 @@ int exynos_gem_map_sgt_with_dma(struct drm_device *drm_dev,
 {
 	int nents;
 
-	mutex_lock(&drm_dev->struct_mutex);
-
 	nents = dma_map_sg(to_dma_dev(drm_dev), sgt->sgl, sgt->nents, dir);
 	if (!nents) {
 		DRM_ERROR("failed to map sgl with dma.\n");
-		mutex_unlock(&drm_dev->struct_mutex);
 		return nents;
 	}
 
-	mutex_unlock(&drm_dev->struct_mutex);
 	return 0;
 }