drm/radeon: Do an MMIO read on interrupts when not uisng MSIs

When not using MSIs, there is no guarantee that DMA from the device
has been fully flushed to point where it's visible to the CPU when
taking an interrupt. To get this guarantee, we need to perform an
MMIO read from the device, which will flush all outstanding DMAs
from bridges between the device and the system.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>