huffyuvdec: trick for plane decoding <= 14bits

Refactor the code to minimize code duplication.

Before:
130870 decicycles in g, 1048139 runs, 437 skips
10bits:  9.048
12bits: 10.733

After:
126960 decicycles in g, 1048136 runs, 440 skips
10bits:  8.642
12bits:  9.656

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>