msm: kgsl: Make the "scratch" global buffer use a random GPU address

Select a random global GPU address for the "scratch" buffer that is used
by the ringbuffer for various tasks.

Change-Id: Ic0dedbaddda71dbf9cb2adab3c6c33a24d6a604c
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>