drm/i915/icl: Refine PG_HYSTERESIS

After doing some measuring, Icelake behaves on a par with Broadwell, and
without having to compromise for low power cores with long latencies, we
can reduce the powergating hysteresis so that the powersaving is enabled
faster. No impact observed on client side throughput measures (so
negligible increase in extra switching), and inspection from high
frequency polling using igt/gem_exec_nop/sequential, provided an estimate
for the upper bound before we can measure a substantial impact on
latency.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191110185806.17413-9-chris@chris-wilson.co.uk