gpu: ion: Secure buffers at allocation time

Currently, buffers allocated as secure must be explicitly secured
with a lock call. Considering all buffers from secured heaps are now
always secured and will never be mapped in the non-secure world,
this extra lock call is redundant. Lock the buffer at allocation
time instead of requiring clients to do so explicitly. Reference
counting will ensure correctness until clients remove the lock/unlock
calls.

Change-Id: I1593b6ec651b13b22171997606c915b8f51a05fd
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>