drm/radeon/kms: wait for cp idle before stopping it.

If we stop CP and that it's still processing thing GPU hang might
happen, this patch wait for CP idle (the wait can timeout) so we
can avoid shutting down CP at bad time. This is especialy usefull
when reseting the GPU as it seems GPU reset fails to properly reset
CP when the CP wasn't stop after being idle.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>