i965/gen7: Use WE_all mode when enabling channel masks for URB write.

Gen7 adds mask bits to the message header for a URB write which allow
the write to apply only to certain channels.  We don't use this
functionality, so to ensure that the entire write always occurs, we
emit an OR instruction to set the mask bits.

With the advent of geometry shaders, URB writes won't just happen at
the end of a thread; they will happen in mid-thread too.  Thus, we can
no longer rely on channel 0 being enabled, so we need to emit the OR
instruction in WE_all mode to ensure that it is executed.