i965: Pass symbolic swizzle to brw_swizzle() as a single argument.

i965: Pass symbolic swizzle to brw_swizzle() as a single argument.

And replace brw_swizzle1() with brw_swizzle().  Seems slightly cleaner
and will allow reusing brw_swizzle() in the vec4 back-end more easily.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>