Blackfin: shutdown video DMA when booting Linux

In case there is no frame buffer driver present in Linux to hand over the
PPI LCD DMA upon boot, the DMA initiated by u-boot to display the splash
screen runs unattended.  Therefore always stop the video driver in u-boot
before starting Linux.  If people don't want this behavior, then they can
simply stub out the video_stop() function in their board video driver.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>