i965/fs: Add split_virtual_grfs and compute_to_mrf after lower_load_payload

If we are going to use LOAD_PAYLOAD operations to fill MRF registers, then
we will need this.

Signed-off-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/mesa/drivers/dri/i965/brw_fs.cpp

@@ -3545,7 +3545,9 @@ fs_visitor::run()
       } while (progress);
 
       if (lower_load_payload()) {
+         split_virtual_grfs();
          register_coalesce();
+         compute_to_mrf();
          dead_code_eliminate();
       }