sm501: Perform a full update after palette change

Changing the palette of a color index has as an immediate effect on
all pixels with the corresponding index on real hardware. Performing a
full update after a palette change is a simple way to emulate this
effect.

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>