egl/dri2: dri2_initialize: Do not reference-count TestOnly display

egl/dri2: dri2_initialize: Do not reference-count TestOnly display

In the case where dri2_initialize is called with a TestOnly display,
the display is not actually initialized, so dri2_egl_display always
fails, and we cannot do any reference counting.

Fixes piglit spec@egl_khr_create_context@verify gl flavor (reproducible
with LIBGL_ALWAYS_SOFTWARE=1).

Fixes: 9ee683f877 (egl/dri2: Add reference count for dri2_egl_display)
Cc: "12.0" <mesa-stable@lists.freedesktop.org>
Reported-by: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>