drm/amdgpu: fix cleaning up reserved VMID on release

We need to reset this or otherwise run into list corruption later on.

Fixes: e44a0fe6 ("drm/amdgpu: rework reserved VMID handling")
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Tested-by: Candice Li <candice.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>