glapi: split the no-op dispatch code into new glapi_nop.c file

This unclutters the glapi.c file a bit.
diff --git a/src/mesa/SConscript b/src/mesa/SConscript
index bdcfffe..ea5bad2 100644
--- a/src/mesa/SConscript
+++ b/src/mesa/SConscript
@@ -251,6 +251,7 @@
 		'main/dispatch.c',
 		'glapi/glapi.c',
 		'glapi/glapi_getproc.c',
+		'glapi/glapi_nop.c',
 		'glapi/glthread.c',
 	]