msm: kgsl: Add EXEC permission to the unassign call

When calling hyp_assign to free a secure buffer we need to pass
in RWX permissions so that when the Hypervisor unmaps the buffer
and maps it back to HLOS, the HLOS is able to use the same
memory for data and instructions.

Change-Id: I71e06a81df85891fecb11c5f197acd54979d2887
Signed-off-by: Shrenuj Bansal <shrenujb@codeaurora.org>