drm/amdgpu: psp_ring_destroy cause psp->km_ring.ring_mem NULL

psp_ring_destroy inside psp_load_fw cause psp->km_ring.ring_mem NULL.
Call Trace occurred when psp_cmd_submit.
should be psp_ring_stop instead.

Reviewed-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Signed-off-by: Wentao Lou <Wentao.Lou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>