drm/radeon: avoid warnings from r600/eg irq handlers on powered off card.

Since we were calling the wptr function before checking if the IH was
even enabled, or the GPU wasn't shutdown, we'd get spam in the logs when
the GPU readback 0xffffffff. This reorders things so we return early
in the no IH and GPU shutdown cases.

Reported-and-tested-by: ManDay on #radeon
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>