drm/amdkfd: Use mmu_notifier_get

We use mmu_notifier_put to free the MMU notifier. That needs to be
paired with mmu_notifier_get to work correctly. Othewrise the next patch
would cause a kernel oops.

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Philip Yang <philip.yang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>