drm/i915/pmu: Use raw clock for rc6 estimation

RC6 is a hardware counter and as such estimating it using the raw clock
during runtime suspend is more appropriate.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
References: 34f43927 ("perf: Add per event clockid support")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20201214094349.3563876-2-tvrtko.ursulin@linux.intel.com