msm: secure_buffer: Update the hyp_assign_phys() api

The hyp_assign_phys() api can be called by different
usecases where it is not guaranteed that the source vm is
always VMID_HLOS.

Pass the responsibility of setting the source_vm to
caller of the function.

Change-Id: I3851a6681f49d4bb6fa5b7a889a16a158497e9e6
Signed-off-by: Neeti Desai <neetid@codeaurora.org>