libvpx

Add sse2 version for vp9_quantize_fp

vp9_quantize_fp is the quantization process used by rtc coding
mode. This commit adds a sse2 implementation of it. The
implementation is modified based on vp9_quantize_b_sse2. No speed
difference from ssse3 version.

Change-Id: I24949c5b27df160b4f35117d28858d269454e64a