egl: Use _eglPreloadDrivers.

Replace the use of _eglPreloadDriver by _eglPreloadDrivers.  The latter
supports EGL_DISPLAY which have a better chance to "just work".
diff --git a/src/egl/main/eglapi.c b/src/egl/main/eglapi.c
index 26e0602..6e8f444 100644
--- a/src/egl/main/eglapi.c
+++ b/src/egl/main/eglapi.c
@@ -101,6 +101,8 @@
 
    drv = disp->Driver;
    if (!drv) {
+      _eglPreloadDrivers();
+
       drv = _eglOpenDriver(disp);
       if (!drv)
          return _eglError(EGL_NOT_INITIALIZED, __FUNCTION__);
@@ -710,9 +712,7 @@
       }
    }
 
-   /* preload a driver if there isn't one */
-   if (!_eglGlobal.NumDrivers)
-      _eglPreloadDriver(NULL);
+   _eglPreloadDrivers();
 
    /* now loop over drivers to query their procs */
    for (i = 0; i < _eglGlobal.NumDrivers; i++) {