drm/i915: use NULL for zero wakeref_t instead of plain integer 0

As of commit 2edc6a75 ("drm/i915: switch intel_wakeref_t underlying
type to struct ref_tracker *") we gained quite a few sparse warnings
about "Using plain integer as NULL pointer" for using 0 to initialize
wakeref_t. Switch to NULL everywhere.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Chaitanya Kumar Borah <chaitanya.kumar.borah@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241002181655.582597-1-jani.nikula@intel.com