AVX2 port of SSSE3 conversion to output 24 bit RGB

Bug: libyuv:778
Test: LibYUVConvertTest.NV21ToRGB24_Opt
Change-Id: I14f7815522d1b790ecd2bb39d9a3441e803b694a
Reviewed-on: https://chromium-review.googlesource.com/953303
Commit-Queue: Frank Barchard <fbarchard@chromium.org>
Reviewed-by: richard winterton <rrwinterton@gmail.com>