drm/radeon: enable RB_ARB before resetting the VCPU

This fixes "UVD not responding, trying to reset the VCPU"
messages on earlier ASICs.

Signed-off-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Merge branch '3.16/uvd-r600' into 3.16/my