X11: fix crash of gnome-shell if mesa is compiled with legacy-x11=dri2

X11 starts up if Mesa is compiled with legacy-x11=dri2 and shows the desktop. It crashes with segfault as soon as an application is started.

At src/egl/drivers/dri2/platform_x11.c function dri2_from_names (at src/gallium/frontends/dri/dri2.c) should be called with offset address instead of with offset value.