mm/hmm: constify hmm_devmem_page_get_drvdata() parameter

Constify pointer parameter to avoid issue when use from code that only
has const struct page pointer to use in the first place.

Link: http://lkml.kernel.org/r/1506972774-10191-1-git-send-email-jglisse@redhat.com
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
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>