drm/atomic: Move enable/connector check up in check_modeset()

Now that handle_conflicting_encoders no longer touches active state,
so there's no need to do the check quite that late any more.

Doing it with all the other checks makes it a lot more clear what the
below block tries to accomplish, and this feels like a better place to
put the check.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1491477543-31257-4-git-send-email-maarten.lankhorst@linux.intel.com