gma500: Fix crash on D2700MUD and various other boards

The recent changes led to the lid timer code being run on various devices.
It does no harm on most but isn't needed. It also calls unconditionally
into the Poulsbo backlight code which goes bang on Cedartrail.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>