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>

src/mesa/drivers/dri/i965/brw_eu.h

@@ -257,6 +257,12 @@ enum brw_urb_write_flags {
     */
    BRW_URB_WRITE_PER_SLOT_OFFSET = 0x10,
 
+   /**
+    * Indicates that the channel masks in the URB_WRITE message header should
+    * not be overridden to 0xff (gen == 7).
+    */
+   BRW_URB_WRITE_USE_CHANNEL_MASKS = 0x20,
+
    /**
     * Convenient combination of flags: end the thread while simultaneously
     * marking the given URB entry as complete.

src/mesa/drivers/dri/i965/brw_eu_emit.c

@@ -2224,7 +2224,7 @@ void brw_urb_WRITE(struct brw_compile *p,
 
    gen6_resolve_implied_move(p, &src0, msg_reg_nr);
 
-   if (brw->gen == 7) {
+   if (brw->gen == 7 && !(flags & BRW_URB_WRITE_USE_CHANNEL_MASKS)) {
       /* Enable Channel Masks in the URB_WRITE_HWORD message header */
       brw_push_insn_state(p);
       brw_set_access_mode(p, BRW_ALIGN_1);