Lets refresh how we track panel self-refresh in autotest to:

1. Unify where code base performs the task.  Currently its at least these places
- client/cros/power/power_utils.py, class DisplayPanelSelfRefresh
- client/site_tests/graphics_Idle/graphics_Idle.py

2. Improve statistics where possible
- currently on intel side perf counter is only available for HSW/BDW

https://chromium.git.corp.google.com/chromiumos/third_party/kernel/+/chromeos-4.4/drivers/gpu/drm/i915/i915_debugfs.c#2570

Perhaps there are other means determine some residency.  

At the very least if we can't calculate the residency stat should instead say 'unknown' or perhaps 'non-zero' if other qualitative indicators let us know there is some residency.  For example,

cat /sys/kernel/debug/dri/0/i915_edp_psr_status
Sink_Support: yes
Source_OK: yes
Enabled: yes
Active: yes
Busy frontbuffer bits: 0x000
Re-enable work scheduled: no
Main link in standby mode: no
HW Enabled & Active bit: yes

Would indicate 'non-zero'.  Presently we return '0' if we can't locate the 'Performance_Counter' string

On arm side, there's only support for rockchip AFAICT.  

https://chromium.git.corp.google.com/chromiumos/third_party/kernel/+/chromeos-4.4/drivers/gpu/drm/rockchip/analogix_dp-rockchip.c#383

We should revisit other ARM SoCs to see if there's potential to expose there as well.