sunxi-disp: Fb_check_var: only check hdmi clock for enabled selections

Currently using vga output can fail, when the other selection output-type is
set to hdmi, because Fb_check_var will check the hdmi clock for any selection
which has its output set to hdmi, even if that selection is not enabled.

This patch fixes this.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Jari Helaakoski <tekkuli@gmail.com>