drm/radeon: fix GUI idle IH debug statements

debug statement for GUI idle interrupt is wrong and incorrectly
reports CP EOP interrupt; trivial issue, but confusing for
someone trying to distinguish interrupt sources while debugging
... fixed

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>