i965/vec4: Fix SSBO stores for 64-bit data

i965/vec4: Fix SSBO stores for 64-bit data

In this case we need to shuffle the 64-bit data before we write it
to memory, source from reg_offset + 1 to write components Z and W
and consider that each DF channel is twice as big.

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

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