Use saturated addition in SSSE3 of 32x32 quant

The 32x32 forward transform can potentially reach peak coefficient
value close to 32700, while the rounding factor can go upto 610.
This could cause overflow issue in the SSSE3 implementation of 32x32
quantization process.

This commit resolves this issue by replacing the addition operations
with saturated addition operations in 32x32 block quantization.

Change-Id: Id6b98996458e16c5b6241338ca113c332bef6e70