ion: Add ion_dma_buf_is_secure() to query buffer type

Outside clients (such as the display or GPU) may import a dmabuf
and need to figure out if it is secure so that it can be handled
accordingly. In an ideal world this would be part of the generic
dma-buf structure but until then we can assume that the only outside
source of dmabuf-ified secure buffers is Ion so add a function to
directly query the status of the buffer from Ion.

ion_dma_buf_is_secure() returns true if the dmabuf was created
by Ion and was created from a secure heap.

Change-Id: Ic0dedbad5e5a4464c6948f05bb7e0e3e94cd4cc2
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>