drm/msm/a6xx: Add inactive_period for a6xx

The target definition for a630 didn't set a reasonable
value for inactive_period so it defaulted to zero and
we were essentially powering down after every submission.
Set it back to the default value to keep the GPU from
bouncing too much during regular workloads.

Signed-off-by: Jordan Crouse <jcrouse@codeaurora.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>