Revert "drm/ast: Support 1600x900 with 108MHz PCLK"

This reverts commit 9bb7b689.

This caused a regression reported to Red Hat.

Fixes: 9bb7b689

 ("drm/ast: Support 1600x900 with 108MHz PCLK")
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220120040527.552068-1-airlied@gmail.com