msm: kgsl: Support global secure memory objects

Currently the few global secure memory objects are allocated in a side
band fashion. Add support to kgsl_allocate_secure_global() to allow
secure objects.

Change-Id: Ic0dedbadc03fb462a8b8bb82a2141f1d5ddd9762
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>