msm: kgsl: Change the secure pagetable initialization sequence

We want to allocate multiple secure global buffers, but we don't
want to store them in an array like we store normal global buffers.
This is because we only ever map the secure global buffers once,
to the secure pagetable at the time of device initialization.
Hence, the secure pagetable needs to be initialized beforehand.
The device specific code must explicitly map/unmap the secure global
buffers during device init/close.

Change-Id: Ic2aa37ca93ab5fc966174493b06f42a873b7d4a0
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>