drm/exynos: Read hpd gpio in is_connected callback

This patch adds a gpio read of hpd during the is_connected
callback. This fixes the case where hdmi is off going into
suspend and the cable is plugged in while suspended. In this
case, the hpd interrupt does not fire and is_connected will
return false.

Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rahul Sharma <Rahul.Sharma@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>