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

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

This reverts commit af13cf609f4257768ad8b80be8cec7f2e6ca8c81, which
appears to cause huge performance problems on Ivybridge.  I'd missed
that the FFTID bits are in the low byte.  The documentation doesn't
indicate that the URB write message header actually wants FFTID - it
just labels those bits as "Reserved."  But it appears necessary.

This does slightly more than revert the original change: originally,
Broadwell had separate code generation, which used MOV, and this patch
only changed it for Gen4-7.  Now that both are unified, reverting this
also makes Broadwell use OR.  Which should be fine.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>