Save NEON registers in VP8 NEON functions

The recent compiler can generate optimized code that uses NEON registers
for various operations besides floating-point operations. Therefore,
only saving callee-saved registers d8 - d15 at the beginning of the
encoder/decoder is not enough anymore. This patch added register saving
code in VP8 NEON functions that use those registers.

Change-Id: Ie9e44f5188cf410990c8aaaac68faceee9dffd31