drm/ast: Support 1600x900 with 108MHz PCLK

[New] Create the setting for 1600x900 @60Hz refresh rate
      by 108MHz pixel-clock.

Signed-off-by: KuoHsiang Chou <kuohsiang_chou@aspeedtech.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20201030074212.22401-1-kuohsiang_chou@aspeedtech.com