i965/vec4: Fix confusion between SWIZZLE and BRW_SWIZZLE macros.

i965/vec4: Fix confusion between SWIZZLE and BRW_SWIZZLE macros.

Most of the VEC4 back-end agrees on src_reg::swizzle being one of the
BRW_SWIZZLE macros defined in brw_reg.h, except in two places where we
use Mesa's SWIZZLE macros.  There is even a doxygen comment saying
that Mesa's macros are the right ones.  They are incompatible swizzle
representations (3 bits vs. 2 bits per component), and the code using
Mesa's works by pure luck.  Fix it.

Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>