drm/amdkfd: Not restore userptr buffer if kfd process has been removed

When kfd process has been terminated not restore userptr buffer after mmu
notifier invalidates a range.

Signed-off-by: Xiaogang Chen <xiaogang.chen@amd.com>
Reviewed-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>