msm: kgsl: Add the per context preemption buffer

This buffer is used by CP to save/restore the VPC data for
the outgoing/incoming context respectively. So allocate and
map it during context initialization and then specify the
gpu address of this buffer in the preemption packets.

Change-Id: I3bb73322848e2f19f1f8e511fa5c303e57898cc8
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>