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/bridge/ti-sn65dsi86.c

@@ -604,13 +604,13 @@ static void ti_sn_bridge_read_valid_rates(struct ti_sn_bridge *pdata,
 		DRM_DEV_ERROR(pdata->dev,
 			      "Unexpected max rate (%#x); assuming 5.4 GHz\n",
 			      (int)dpcd_val);
-		/* fall through */
+		fallthrough;
 	case DP_LINK_BW_5_4:
 		rate_valid[7] = 1;
-		/* fall through */
+		fallthrough;
 	case DP_LINK_BW_2_7:
 		rate_valid[4] = 1;
-		/* fall through */
+		fallthrough;
 	case DP_LINK_BW_1_62:
 		rate_valid[1] = 1;
 		break;