drm/radeon: init lockup timeout on ring init

Reset the lockup timeout on ring (re-)initialisation.

Otherwise we get error messages like this on gpu resets:
[ 1559.949177] radeon 0000:01:00.0: GPU lockup CP stall for more than 1482270msec

Signed-off-by: Christian König <deathsimple@vodafone.de>
cc: stable@vger.kernel.org
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>