drm/amdgpu:use work instead of delay-work

no need to use a delay work since we don't know how
much time hypervisor takes on FLR, so just polling
and waiting in a work.

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Xiangliang Yu <Xiangliang.Yu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>