sunxi: video: Force h/vsync active high when using ext. vga dac on some boards

On both my A13-OLinuxIno and my A13-OLinuxIno-Micro, the vga output gives an
unstable image when active low v or hsync is used.

The problem seems to be specific to the OLinuxIno A13 (normal & micro)
boards. I've just looked up the schematics and they use an opendrain driver
for the vga sync lines, and with sync pulses it is the logical high->low
edge of the pulse which counts for the timing, which with an active low
sync is being driven by the pull-up, and that simply seems to not drive
it hard enough to get a stable image.

So force v and hsync active high on these boards. independent of what the
modeline says. This fixes the unstable image.

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