drm/amdgpu: set the VM pointer to NULL in amdgpu_job_prepare

As soon as the prepare phase is completed the VM might be released,
better set it to NULL.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>