disk cache: initialize cache path and index only when used

disk cache: initialize cache path and index only when used

This patch makes disk_cache initialize path and index lazily so
that we can utilize disk_cache without a path using callback
functionality introduced by next patch.

v2: unmap mmap and destroy queue only if index_mmap exists

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>