i965/vec4: fix scratch writes for 64bit data

i965/vec4: fix scratch writes for 64bit data

Mostly the same stuff as usual: we ned to shuffle the data before we
write and we need to emit two 32-bit write messages (with appropriate
32-bit writemask channels set) for a full dvec4 scratch write.

v2: use byte_offset() instead of offset().

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