drm/amdgpu:fix waiting on dirty fence

if bo->shadow is NULL (race issue:BO shadow was just released
and gpu-reset kick in but BO hasn't yet) recover_vram_from_shadow
won't set @next, so the following "fence=next"
will wrongly use a fence pointer which may already dirty.
fixing it by set next to NULL prior to recover_vram_from_shadow

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Chunming <Zhou&lt;david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>