intel/compiler: Use generic SEND for Gen7+ FB writes

This takes care of generate_fb_write/fire_fb_write/brw_fb_WRITE's stuff
earlier in the visitor.  It will also make it easier to generate SENDSC
messages with indirect extended descriptors in a few patches.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>

src/intel/compiler/brw_schedule_instructions.cpp

@@ -412,6 +412,11 @@ schedule_node::set_latency_gen7(bool is_haswell)
             latency = 14000;
             break;
 
+         case GEN6_DATAPORT_WRITE_MESSAGE_RENDER_TARGET_WRITE:
+            /* completely fabricated number */
+            latency = 600;
+            break;
+
          default:
             unreachable("Unknown render cache message");
          }