egl_g3d: don't drop single-buffered configs

This is needed to be able to use EGL on any existing X window, and
seems a good idea in general,

Rejecting single-buffered configs should be done in EGL itself if
necessary, and not in the native API.

src/gallium/state_trackers/egl/x11/native_dri2.c

@@ -518,10 +518,6 @@ dri2_display_convert_config(struct native_display *ndpy,
    if (!(mode->renderType & GLX_RGBA_BIT) || !mode->rgbMode)
       return FALSE;
 
-   /* skip single-buffered configs */
-   if (!mode->doubleBufferMode)
-      return FALSE;
-
    /* only interested in native renderable configs */
    if (!mode->xRenderable || !mode->drawableType)
       return FALSE;