gma500: Fix backlight crash

Initial changes to get backlight behaviour we want and to fix backlight crashes
on suspend/resume paths.

[Note: on some boxes this will now produce a warning about the backlight, this
 isn't a regression it's an unfixed but non harmful case I still need to nail]

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