msm: ion: Update the permissions while making assign call

When freeing a secure buffer, the RWX permissions need to be
passed as a parameter to the hyp_assign call. This will ensure
that when the hypervisor unmaps the buffer, HLOS can use it
for data as well as instructions.

Change-Id: I2f1e67dda8f3ebedee552a2908b7f57c18754d42
Signed-off-by: Neeti Desai <neetid@codeaurora.org>