i965/gen7: Add the ability to send URB_WRITE_OWORD messages.

i965/gen7: Add the ability to send URB_WRITE_OWORD messages.

Previously, brw_urb_WRITE() would always generate a URB_WRITE_HWORD
message, we always wanted to write data to the URB in pairs of varying
slots or larger (an HWORD is 32 bytes, which is 2 varying slots).

In order to support geometry shader EndPrimitive functionality, we'll
need the ability to write to just a single OWORD (16 byte) slot, since
we'll only be outputting 32 of the control data bits at a time.  So
this patch adds a flag that will cause brw_urb_WRITE to generate a
URB_WRITE_OWORD message.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>