gma500: Prevent endless loop in panel power up sequence

Some devices don't have a panel connected to LVDS and thus will never power up.
This patch checks the power sequence progress bits in PP_STATUS to prevent an
endless loop on such devices.

Signed-off-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>