fbdev: vga16fb: fix orig_video_isVGA confusion

At some point the orig_video_isVGA field of screen_info was repurposed
for video type. Using it directly for video type check is unsafe as it can
still mean yes (1) or no-output (0) in certain configurations. I had one of those.

Signed-off-by: Zsolt Kajtar <soci@c64.rulez.org>
Signed-off-by: Helge Deller <deller@gmx.de>