sm501: Fix support for non-zero frame buffer start address

Display updates and drawing hardware cursor did not work when frame
buffer address was non-zero. Fix this by taking the frame buffer
address into account in these cases. This fixes screen dragging on
AmigaOS. Based on patch by Sebastian Bauer.

Signed-off-by: Sebastian Bauer <mail@sebastianbauer.info>
Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>