drm/radeon/kms: r600 handle irq vector ring overflow

In some rare case i faced an irq overflow quickly followed by
a GPU lockup (hard hang) this patch try to deal with irq vector
ring overflow, so far haven't been able to reproduce it with
the patch.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@linux.ie>