drm/amdgpu: fix gfx VF FLR test fail on navi

Cp wptr in wb buffer is outdated after VF FLR.
The outdated wptr may cause cp to execute unexpected packets.
Reset cp wptr in wb buffer.

Signed-off-by: HaiJun Chang <HaiJun.Chang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>