Properly enable S32_D16_filter_DX_SSE2 optimization.

Properly enable S32_D16_filter_DX_SSE2 optimization.

Currently, the S32_D16_filter_DX_SSE2 optimization is only used in
configurations where the maximum SSE level is SSE2.
This patch enables it for higher levels, as well as fixing a color
conversion bug when the subpixels are converted into RGB565 format.
Also, refactored the function a bit, to make future modifications
less error-prone.

Author: henrik.smiding@intel.com

Signed-off-by: Henrik Smiding <henrik.smiding@intel.com>
Committed: http://code.google.com/p/skia/source/detail?r=14333

R=reed@google.com, mtklein@google.com, tomhudson@google.com, djsollen@google.com, joakim.landberg@intel.com

Author: henrik.smiding@intel.com

Review URL: https://codereview.chromium.org/239453010

git-svn-id: http://skia.googlecode.com/svn/trunk@14403 2bbb7eff-a529-9590-31e7-b0007b416f81