drm/i915/pmu: Check actual RC6 status

RC6 support cannot be simply established by looking at the static device
HAS_RC6() flag. There are cases which disable RC6 at driver load time so
use the status of those check when deciding whether to enumerate the rc6
counter.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reported-by: Eero T Tamminen <eero.t.tamminen@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210331101850.2582027-1-tvrtko.ursulin@linux.intel.com