drm/amdgpu:use clear_ring to clr RB

In resume routine, we need clr RB prior to the
ring test of engine, otherwise some engine hang
duplicated during GPU reset.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>