egl: fix _EGL_NATIVE_PLATFORM fallback

When the X11 or Haiku platforms were compiled in, they would bypass the
`_EGL_NATIVE_PLATFORM` fallback by always returning themselves instead.

Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 86d3a346)