drm/ast: Fixed 1920x1080 sync. polarity issue

[Bug] Change the vertical synchroous polary of 1920x1080 @60Hz
      from  Negtive to Positive

Signed-off-by: KuoHsiang Chou <kuohsiang_chou@aspeedtech.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201105094729.106059-1-kuohsiang_chou@aspeedtech.com