intel/compiler: Set "Null Render Target" ex_desc bit on Gen11

When there are no color regions (i.e. a depth only pass), we can set
the "Null Render Target" bit in the Gen11 RT write extended message
descriptor to indicate that it should behave as if it's writing to a
null render target, without the need for a binding table entry.

This lets drivers avoid setting up that null RT binding table entry,
but more importantly means the HW doesn't actually have to bother
looking up the surface state.

Together with the next patch, this improves performance in Car Chase on
an Icelake 8x8 (locked to 700Mhz) by 0.0445526% +/- 0.0132736% (n=832).

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>

src/intel/compiler/brw_fs.cpp

@@ -4494,6 +4494,9 @@ lower_fb_write_logical_send(const fs_builder &bld, fs_inst *inst,
           * in the extended message descriptor, in lieu of using a header.
           */
          ex_desc = inst->target << 12 | src0_alpha_present << 15;
+
+         if (key->nr_color_regions == 0)
+            ex_desc |= 1 << 20; /* Null Render Target */
       }
 
       inst->opcode = SHADER_OPCODE_SEND;