[media] staging: atomisp: Fix endless recursion in hmm_init

hmm_init calls hmm_alloc to set dummy_ptr, hmm_alloc calls
hmm_init when dummy_ptr is not yet set, which is the case in
the call from hmm_init, so it calls hmm_init again, this continues
until we have a stack overflow due to the recursion.

This commit fixes this by adding a separate flag for tracking if
hmm_init has been called. Not pretty, but it gets the job done,
eventually we should be able to remove the hmm_init call from
hmm_alloc.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>