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/tegra/dc.c

@@ -135,7 +135,7 @@ static inline u32 compute_dda_inc(unsigned int in, unsigned int out, bool v,
 
 		default:
 			WARN_ON_ONCE(1);
-			/* fallthrough */
+			fallthrough;
 		case 4:
 			max = 4;
 			break;