drm/i915: Rename intel_wakeref_[is]_active

Our general rule is to use is/has as the verb for boolean functions,
rename intel_wakeref_active to intel_wakeref_is_active so the question
being asked is clear.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190625130128.11009-6-chris@chris-wilson.co.uk