msm: secure_buffer: Fix possible NULL pointer dereference

Presently we were passing NULL dma_handle while
making dma_alloc_coherent call. This is an issue
as alloc function would try to populate handle
value with dma address and would end up getting
NULL pointer error. Fix this.

Change-Id: I38ebb9bbae2d6e1e1311fe438b3c8fc3f65e5692
Signed-off-by: Susheel Khiani <skhiani@codeaurora.org>