drm/i915: fix operator precedence when enabling RC6p

As noticed by Torsten Kaiser, the operator precedence can play tricks with
us here.

CC: Dave Airlie <airlied@redhat.com>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>