Commit 6807874d "sun4m: obey -vga none" changed the sun4m machines so that
they could be started without a framebuffer installed, but as no default
display type was configured the machines would start in headless mode without
an explict -vga option.

Set the default display type for all sun4m machines to TCX so that they will
start with a framebuffer if one is not specifically requested.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>