msm: kgsl: Fix the snapshot mempool size calculation

The snapshot mempool size takes into account the memory
required for section headers. It was being calculated based
on old header structure. Update that to avoid corruption/
buffer overflow of the mempool memory.

Change-Id: I07274934e4c0dced707e03be3e31b2459e00d706
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>