msm: kgsl: Increase the global memory size

This is needed to accommodate the preemption buffers
for a6xx. The per ringbuffer preemption buffer is
little more than 2 mb, so increase the size to 20 mb.

Change-Id: I968cb69c3b023f97a43ddb5e07c3a361bec06b2a
Signed-off-by: Harshdeep Dhatt <hdhatt@codeaurora.org>