arm: mx6: cm-fx6: setup hdmi only on hdmi enable

Refactor display code to only setup hdmi if do_enable_hdmi() is invoked.

Cc: Stefano Babic <sbabic@denx.de>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>