drm/amdgpu: reset psp ring wptr during ring_create

[Why]
psp ring wptr is not initialized properly in ring_create,
which would lead to psp failure after several gpu reset.

[How]
Set ring_wptr to zero in psp_ring_create.

Signed-off-by: YuBiao Wang <YuBiao.Wang@amd.com>
Reviewed-by: Horace Chen <horace.chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>