drm/amdgpu/sriov:Correct pfvf exchange logic

The pfvf exchange need be in exclusive mode. And add pfvf exchange in gpu
reset.

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-By: Xiangliang Yu <Xiangliang.Yu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>