NEON FDCT updated to match current C code

- Removed fast_fdct4x4_neon and fast_fdct8x4_neon
- Uses now short_fdct4x4 and short_fdct8x4
- Gives ~1-2% speed-up on Cortex-A8/A9

Change-Id: Ib62f2cb2080ae719f8fa1d518a3a5e71278a41ec