va: Fix 4:2:2 jpeg decoding

1. enhance the jpeg decoder template caps by removing those subsamplings without reported pixel formats (useful for mesa gallium)
2. add y42b color format for 4:2:2
3. handle 4:2:2 for i965 with nv12 (by using its internal color conversion)
4. fixed a potential critical message when a non-valid caps configuration is reached.
5. fixed a couple mem leaks.

Fixes: #1378 (closed)