drm/amdgpu: Log after a successful ring reset

When a ring reset happens, the kernel log shows only "amdgpu: Starting
<ring name> ring reset", but when it finishes nothing appears in the
log. Explicitly write in the log that the reset has finished correctly.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>