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/xen/xen_drm_front.c

@@ -649,9 +649,7 @@ static void displback_changed(struct xenbus_device *xb_dev,
 
 	switch (backend_state) {
 	case XenbusStateReconfiguring:
-		/* fall through */
 	case XenbusStateReconfigured:
-		/* fall through */
 	case XenbusStateInitialised:
 		break;
 
@@ -701,7 +699,6 @@ static void displback_changed(struct xenbus_device *xb_dev,
 		break;
 
 	case XenbusStateUnknown:
-		/* fall through */
 	case XenbusStateClosed:
 		if (xb_dev->state == XenbusStateClosed)
 			break;