drm/ast: Fix video broken in some resolutions on ast2600

Some cases are not handled well for ast2600.

Signed-off-by: Jammy Huang <jammy_huang@aspeedtech.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220916091706.4559-1-jammy_huang@aspeedtech.com