[Why]
when vram lost happened in guest, try to write vram can lead to
kernel stuck.

[How]
When the readback data is invalid, don't do write work, directly
reschedule a new work.
Signed-off-by: Jingwen Chen <Jingwen.Chen2@amd.com>
Reviewed-by: Monk Liu<monk.liu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>