Early termination for chroma encoding

Early termination for chroma encoding
Faster chroma encoding by terminating early if heuristics indicate that the block will be DC-only.
This works because the vast majority of inter chroma blocks have no coefficients at all, and those that do are almost always DC-only.
Add two new helper DSP functions for this: dct_dc_8x8 and var2_8x8.  mmx/sse2/ssse3 versions of each.
Early termination is disabled at very low QPs due to it not being useful there.
Performance increase is ~1-2% without trellis, up to 5-6% with trellis=2.
Increase is greater with lower bitrates.