drm/amdgpu: if a GDS switch is needed emit a pipeline sync as well

Otherwise we might change the GDS settings while they are still in use.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Chunming Zhou <david1.zhou@amd.com>