video/fbdev: refactor video= cmdline parsing

Make the video_setup() function slightly easier to read by removing the
repeated checks for !global. Remove the misleading return value comment
while at it.

I'm slightly hesitant to change any of this, but here goes anyway, with
hopes that the next person to have to look at this has it a wee bit
easier.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>