egl/dri2: Fix include path of u_atomic.h introduced e7e29189

This was causing a failure to build on SCons due to a missing
-Isrc/egl. Instead of adding in that path, lets just -Isrc/
and include "utils/u_atomic.h".

Reviewed-by: Matt Turner <mattst88@gmail.com>
diff --git a/src/egl/drivers/dri2/egl_dri2.c b/src/egl/drivers/dri2/egl_dri2.c
index 0290c07..461735f 100644
--- a/src/egl/drivers/dri2/egl_dri2.c
+++ b/src/egl/drivers/dri2/egl_dri2.c
@@ -52,7 +52,7 @@
 #endif
 
 #include "egl_dri2.h"
-#include "../util/u_atomic.h"
+#include "util/u_atomic.h"
 
 /* The kernel header drm_fourcc.h defines the DRM formats below.  We duplicate
  * some of the definitions here so that building Mesa won't bleeding-edge