egl: directly access static members instead of using _egl{Get,Set}ConfigKey()

egl: directly access static members instead of using _egl{Get,Set}ConfigKey()

This function is meant for when the attribute is unknown at compile-time
(eg. user-specified), but in all these cases it is much simpler to just
read/write the member directly.

Suggested-by: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3816>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3816>