gma500: introduce some register maps

All the conditional ugly register selection really wants to be
cleaned up. Use a struct describing each pipe and its registers.

This will also let us hide some of the oddments between platforms
for any future merging of bits together. In particular the way the
DPLL and FP registers randomly wander around.

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