i965: Write at least some data in SIMD8 URB write messages.

i965: Write at least some data in SIMD8 URB write messages.

According to the "URB SIMD8 Write > Write Data Payload" documentation,
"The write data payload can be between 1 and 8 message phases long."

Apparently, the simulator considers it an error if you issue an URB
SIMD8 message with only a header and no actual data to write.

v2: Try to put in a better PRM citation, now that the Broadwell docs
    actually exist (requested by Jordan).

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>