drm/gma500: Check connector status before restoring sdvo

This patch fixes a bug introduced by:

commit 749387dc
Author: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Date:   Sun Apr 7 16:35:50 2013 +0200
drm/gma500: Fix hibernation problems on sdvo encoders

The bug is triggered when we do a mode set on a sdvo encoder with all
connectors in the disconnected state. A crtc is considered enabled by
drm even though all of its connectors are disconnected. Work around
this by adding a check in our sdvo restore function.

Also remove the unneeded dpms on. Prepare and Commit will take care of
that.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>