drm/i915: Remove PSR Perf Counter for SKL+

Whenever DMC firmware put the HW into DC State a bunch
of registers including this perf counter is reset to 0.

Even with PSR active and working we could still read
"Performance_Counter: 0" what will misslead people to believe
PSR is broken. For instance on SKL we can only see PC10
residency with screen on if PSR is working properly.
However Performance_Counter was showing 0.

Even if it restored properly on DC6 exit we don't want to
give users the wrong impression that PSR is not working
while we know for sure it is.

So, it is better to remove this counter information while
we don't have a better way to track PSR residency.

Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Durgadoss R <durgadoss.r@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>