drm/i915/pmu: Remove conditional HOTPLUG_CPU registration

Even for static CPU configurations, the hotplug CPU framework is still
used to determine the CPU topology, and is still being used by the perf
event register to check for valid CPUs.

Fixes: b46a33e2

 ("drm/i915/pmu: Expose a PMU interface for perf queries")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Dmitry Rogozhkin <dmitry.v.rogozhkin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171123123432.25035-1-tvrtko.ursulin@linux.intel.com