i965/vec4: Use MOV, not OR, to set URB write channel mask bits.

i965/vec4: Use MOV, not OR, to set URB write channel mask bits.

g0.5 has nothing of value to contribute to m0.5.  In both the VS and GS
payload, g0.5 contains the scratch space pointer - which is definitely
not of any use.  The GS payload also contains FFTID, but the URB write
message header doesn't want FFTID.

The only reason I used OR was because Eric originally requested it.
On Broadwell, I used MOV, and that's worked out fine.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Matt Turner <mattst88@gmail.com>