msm: kgsl: Use per-target scratch memory for preemption

a5xx and a6xx both have a need for a bit of scratch memory for preemption.
Rename the existing "counters" member in struct adreno_preemption from a5xx
and use it for a6xx to store the context restore address.

Change-Id: Ic0dedbad6493e99b35d8c771673e8f9f686f927c
Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>