gma500: Replace SDVO code with slightly modified version from i915

Our current SDVO implementation is not working properly, so replace it with
a modified version of the i915. Further testing and debugging is needed to make
sure we can handle the different SDVO setups and wiring.

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>