haven: Fix NULL pointer dereference in hh_rm_populate_mem_attr_desc()

Use the locally calculated value of memory attribute entries when
populating the memory attribute descriptor, instead of always
dereferencing the client provided memory attribute descriptor,
as clients may not always provide one.

Change-Id: Ifbe586336e69e439ed404067616cce7515fd7953
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>