drivers: qcom: secure_buffer: Optimize by re-using the same buffer

Re-use the same buffer for the hyp assign calls rather than allocating
a new one for every single call.

Change-Id: Ia76f9743c7eb83d3432ac19cc98be243434c2aa8
Signed-off-by: Rohit Vaswani <rvaswani@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>