altos/stm-vga: Fix DMA reset to load scanline each time

If we load the scanline register while DMA is running, it doesn't
actually get reloaded until after the first transfer from the next
line, leaving a weird jog in the middle of the screen.

Also flip to SPI1, as Bdale is using that on the 1802 board.

Signed-off-by: Keith Packard <keithp@keithp.com>