drm/amdgpu: fix check in amdgpu_hmm_invalidate_gfx

drm/amdgpu: fix check in amdgpu_hmm_invalidate_gfx

For a short moment during alloc/free the userptr BO is not part of his VM,
so bo->vm_bo can be NULL.

Keep a reference to the VM root PD as parent of the userptr BO so that
we can always use that to wait for all submissions of the VM instead of
only the one involving the userptr BO.

Signed-off-by: Christian König <christian.koenig@amd.com>
Fixes: 91250893cbaa ("drm/amdgpu: fix waiting for all submissions for userptrs")
Closes: https://gitlab.freedesktop.org/drm/amd/-/work_items/5399
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
(cherry picked from commit 631849ff5d603841e74f19f4a5e30fe1f7d7cf30)
Cc: stable@vger.kernel.org