gpu: ion: Make secure buffers 1MB chunks

Secure buffers have a requirement to be generated in 1MB
chunks. Change the sg_table generated to be an sg list of
1MB chunks of the contiguous buffer if the secure flag
is indicated.

Change-Id: Ia03d283cddfb978fc9e15975c2d80f138269b5ce
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>