drm/amdkfd: Correct mmu_notifier_get failure handling

If process has signal pending, mmu_notifier_get_locked fails and calls
ops->free_notifier, kfd_process_free_notifier will schedule
kfd_process_wq_release as process refcount is 1, but process structure
is already freed. This use after free bug causes system crash with
different backtrace.

The fix is to increase process refcount and then decrease the refcount
after mmu_notifier_get success.

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