cdgraphics: fix incorrect vertical offset mask in cdg_scroll()

The vertical offset mask 0x07 is suspicious.

    v_off = FFMIN(data[2] & 0x07, CDG_BORDER_HEIGHT - 1);

Note that v_off is up to 11 (CDG_BORDER_HEIGHT - 1), the correct mask
should be 0x0F.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>