drm: Catch stop possible NULL pointer reference

This was caught by Weiss. Also added some comments to the
fb_changed and mode_changed variables to explain what they do.

Signed-off-by: Jakob Bornecrantz <jakob@vmware.com>
Tested-by: Thomas White <taw@bitwiz.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>