egl/dri2: fix segfault when display initialisation fails

egl/dri2: fix segfault when display initialisation fails

dri2_display_destroy() is called when platform specific display
initialisation fails. However, this would typically lead to a
segfault due to the dri2_egl_display vbtl not having been set up.

Fixes: 2db95482964 ("loader_dri3/glx/egl: Optionally use a blit
context for blitting operations")
Signed-off-by: Frank Binns <francisbinns@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>