i965: Use MOV, not OR for setting URB write channel enables on Gen8+.

i965: Use MOV, not OR for setting URB write channel enables on Gen8+.

On Broadwell, g0.5 contains the "Scratch Space Pointer"; using OR
puts some bits of that into "ignored" sections of our message header.

While this doesn't hurt, it's also not terribly /useful/.  Using MOV
is sufficient to set the only interesting bits in this part of the
message header.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>