msm: kgsl: Write reference timestamp to unique memory only for A5xx

To handle timestamp interrupt storm we write a reference timestamp
into unique memory for each submission. This problem is present only
on A5xx GPUs, so don't write this reference timestamp on GPUs other
than A5XX.

Change-Id: I43530035a2266bb6cfd5b35dc2d2064e25a3260e
Signed-off-by: Hareesh Gundu <hareeshg@codeaurora.org>