i965: Provide sse2 version for rgba8 <-> bgra8 swizzle

i965: Provide sse2 version for rgba8 <-> bgra8 swizzle

The existing code used ssse3, and because it isn't compiled in a separate
file compiled with that, it is usually not used (that, of course, could
be fixed...), whereas sse2 is always present at least with 64bit builds.
This should be pretty much as fast as the pshufb version, albeit those
code paths aren't really used on chips without llc in any case.

v2: fix andnot argument order, add comments
v3: use pshuflw/hw instead of shifts (suggested by Matt Turner), cut comments

Reviewed-by: Matt Turner <mattst88@gmail.com>