intel: Flatten the makefile structure

This pulls isl and genxml into a single make file so that they can properly
build in parallel.  This isn't terribly important now as genxml just
generates sources which happens serially first anyway but it will be more
important as we add more stuff to src/intel.

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

configure.ac

@@ -2746,8 +2746,6 @@ AC_CONFIG_FILES([Makefile
 		src/glx/tests/Makefile
 		src/gtest/Makefile
 		src/intel/Makefile
-		src/intel/genxml/Makefile
-		src/intel/isl/Makefile
 		src/intel/tools/Makefile
 		src/intel/vulkan/Makefile
 		src/loader/Makefile