brw: avoid invalid URB messages

Some new CTS tests have geometry shader looking like this :

   void main()
   {
      gl_Position = gl_in[0].gl_Position;
      EmitVertex();
      EndPrimitive();
      // <-- some storage buffer write
   }

The generate shader has :
   - a message to write the position
   - a message to write to the storage buffer
   - a final message to end the thread

This generates an empty EOT URB messages which is apparently not legal
(simulation complains, HW hangs) :

send(8)         nullUD          g126UD          nullUD          0x04088007                0x00000000
                urb MsgDesc: offset 0 SIMD8 write masked  mlen 2 ex_mlen 0 rlen 0 { align1 1Q A@1 EOT };

Instead emit a write with actual data and the mask set at 0 to discard
the effect :

mov(8)          g127<1>UD       0x00000000UD                    { align1 WE_all 1Q };
mov(8)          g125<1>UD       0x00000000UD                    { align1 1Q };
send(8)         nullUD          g126UD          g125UD          0x04088007                0x00000040
                urb MsgDesc: offset 0 SIMD8 write masked  mlen 2 ex_mlen 1 rlen 0 { align1 1Q A@1 EOT };

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: mesa-stable
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/38243>

src/intel/compiler/brw/brw_compile_gs.cpp

@@ -52,8 +52,10 @@ brw_emit_gs_thread_end(brw_shader &s)
 
       brw_reg srcs[URB_LOGICAL_NUM_SRCS];
       srcs[URB_LOGICAL_SRC_HANDLE] = s.gs_payload().urb_handles;
+      srcs[URB_LOGICAL_SRC_DATA] = brw_imm_ud(0);
+      srcs[URB_LOGICAL_SRC_CHANNEL_MASK] = brw_imm_ud(0);
       urb = abld.URB_WRITE(srcs, ARRAY_SIZE(srcs));
-      urb->components = 0;
+      urb->components = 1;
    } else {
       brw_reg srcs[URB_LOGICAL_NUM_SRCS];
       srcs[URB_LOGICAL_SRC_HANDLE] = s.gs_payload().urb_handles;