treewide: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1]. Also, remove unnecessary
fall-through markings when it is the case.

[1] https://www.kernel.org/doc/html/v5.7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-through

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/gpu/drm/drm_bufs.c

@@ -537,7 +537,7 @@ int drm_legacy_rmmap_locked(struct drm_device *dev, struct drm_local_map *map)
 	switch (map->type) {
 	case _DRM_REGISTERS:
 		iounmap(map->handle);
-		/* FALLTHROUGH */
+		fallthrough;
 	case _DRM_FRAME_BUFFER:
 		arch_phys_wc_del(map->mtrr);
 		break;