drm/radeon/r420: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Addresses-Coverity-ID: 1357317 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

drivers/gpu/drm/radeon/r420.c

@@ -109,6 +109,7 @@ void r420_pipes_init(struct radeon_device *rdev)
 	default:
 		/* force to 1 pipe */
 		num_pipes = 1;
+		/* fall through */
 	case 1:
 		tmp = (0 << 1);
 		break;