drm/amdgpu: initialize bo_va_list when add gws to process

bo_va_list is list_head, so initialize it.

Signed-off-by: xinhui pan <xinhui.pan@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>