gma500: Make VGA and HDMI connector hotpluggable

Both VGA and HDMI connectors are available on my Asus EeePC X101CH.
This patch will cause output to be shown on either when plugged in.
For both, it shows the leftmost 800x600, of the 1024x600 on LVDS.

Signed-off-by: Kero van Gelder <kero@chello.nl>
Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>