video: Allow driver to specify the line length

At present line_length is always calculated in video_post_probe(). But
some hardware may use a different line length, e.g. with a 1366-wide
display.

Allow the driver to set this value if needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Anatolij Gustschin <agust@denx.de>