msm: kgsl: Do not allocate scratch memory for A3xx

A3xx device gets the ring buffer read pointer directly
from the GPU registers. So don’t allocate scratch memory
which can’t be used for A3xx GPU devices.

Change-Id: I95016dfc169b9fee74e978f5560592740f34515e
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>