i965/gen7: Allow URB_WRITE channel masks to be used.

i965/gen7: Allow URB_WRITE channel masks to be used.

Previously, brw_urb_WRITE() would unconditionally override the channel
masks in the URB_WRITE message to 0xff (indicating that all channels
should be written to the URB).

In order to support geometry shader EndPrimitive functionality, we'll
need the ability to set the channel masks programatically, so that we
can output just 32 of the control data bits at a time.  So this patch
adds a flag that will prevent brw_urb_WRITE() from overriding them.

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