SSE2 optimisation for quantize in high bit depth

When configured with high bit detpth enabled, the 8bit quantize
function stopped using optimised code. This made 8bit content
decode slowly. This commit re-enables the SSE2 optimisation
(but not the SSSE3 optimisation).

Change-Id: Id015fe3c1c44580a4bff3f4bd985170f2806a9d9