gma500: Initial support for our encoder and connector structs

First step towards adding i915 alike encoder and connector abstractions. This
will make life easier when adding i915 output code into our driver. It also
removes the old psb_intel_output struct.

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>