drm/radeon/kms: fix resume regression for some r5xx laptops

I had removed this when I switched the atom indirect io methods
to use the io bar rather than the mmio bar, but it appears it's
still needed.

Reported-by: Mark Lord <kernel@teksavvy.com>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Dave Airlie <airlied@redhat.com>