egl/display: only detect the platform once

egl/display: only detect the platform once

My refactor missed the fact that `native_platform` is static.
Add the proper guard around the detection code, as it might not be
necessary, and only print the debug message when a detection was
actually performed.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=101252
Fixes: 7adb9b094894a512c019 ("egl/display: remove unnecessary code and
                              make it easier to read")
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Grazvydas Ignotas <notasas@gmail.com>
Acked-by: Emil Velikov <emil.l.velikov@gmail.com>