mm/hmm: fix build when HMM is disabled

Combinatorial Kconfig is painfull. Withi this patch all below combination
build.

1)

2)
CONFIG_HMM_MIRROR=y

3)
CONFIG_DEVICE_PRIVATE=y

4)
CONFIG_DEVICE_PUBLIC=y

5)
CONFIG_HMM_MIRROR=y
CONFIG_DEVICE_PUBLIC=y

6)
CONFIG_HMM_MIRROR=y
CONFIG_DEVICE_PRIVATE=y

7)
CONFIG_DEVICE_PRIVATE=y
CONFIG_DEVICE_PUBLIC=y

8)
CONFIG_HMM_MIRROR=y
CONFIG_DEVICE_PRIVATE=y
CONFIG_DEVICE_PUBLIC=y

Link: http://lkml.kernel.org/r/20170826002149.20919-1-jglisse@redhat.com
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>