video: fbdev: pm2fb: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a fallthrough pseudo-keyword.

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/0eedb3972a0032da4997a2a47cf0665fbe9c56ca.1605896060.git.gustavoars@kernel.org

drivers/video/fbdev/pm2fb.c

@@ -239,6 +239,7 @@ static u32 to3264(u32 timing, int bpp, int is64)
 		fallthrough;
 	case 16:
 		timing >>= 1;
+		fallthrough;
 	case 32:
 		break;
 	}