sunxi: video: Remove sunxi_display.enabled variable

Having both a sunxi_display.enabled variable and
sunxi_display.monitor == sunxi_monitor_none duplicates state, use
sunxi_display.monitor = sunxi_monitor_none when ever we do not have a display.

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