fb: pvr2fb: Shared IRQ for dreamcast pvr2.

The maple bus driver (http://lkml.org/lkml/2007/9/4/165) uses hardware
synchronisation between the maple bus and the VBLANK to poll the maple
bus. This patch makes the interrupt shareable.

By definition the interrupt is for both devices.

Signed-off by: Adrian McMenamin <adrian@mcmen.demon.co.uk>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>