drm/i915/pmu: fix noderef.cocci warnings

drivers/gpu/drm/i915/i915_pmu.c:795:34-40: ERROR: application of sizeof to pointer

 sizeof when applied to a pointer typed expression gives the size of
 the pointer

Generated by: scripts/coccinelle/misc/noderef.cocci

Fixes: 109ec558 ("drm/i915/pmu: Only enumerate available counters in sysfs")
Signed-off-by: Fengguang Wu <fengguang.wu@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/20180112170340.5387-1-tvrtko.ursulin@linux.intel.com