sunxi: display: Replace #ifdef-ery with helper functions

All the #ifdef-ery in selecting the default and fallback monitor type is
becoming unyielding and makes the code hard to read, replace it with a few
helper functions.

This will also be useful with the upcoming CHIP board which has display
adapter daughterboards which should be runtime detectable.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>