drv_video_init(): simplify logic

Simplify nesting of drv_video_init() and use a consistent way of
indicating failure / success. Before, it took me some time to realize
which of the returns was due to an error condition and which of them
indicated success.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>