drm/msm: Allocate secure buffer objects

Allow the user to allocate and use secured buffer objects. Secured
buffer objects are suitable for use as a write target while the GPU
is in secure mode. They work exactly like regular buffers except
Secure buffers cannot be mmap()ed.

Change-Id: Ic0dedbadd8135fd8472b38ddf61e2bc70983b12f
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>