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/omapdrm/dss/venc.c

@@ -597,7 +597,7 @@ static void venc_bridge_mode_set(struct drm_bridge *bridge,
 	switch (venc_mode) {
 	default:
 		WARN_ON_ONCE(1);
-		/* Fall-through */
+		fallthrough;
 	case VENC_MODE_PAL:
 		venc->config = &venc_config_pal_trm;
 		break;