drm/i915: fix sparse warning for using false as NULL

drivers/gpu/drm/i915/intel_dpll_mgr.c:1200:32: warning: Using plain integer as NULL pointer

Fixes: 304b65cb ("drm/i915: Move SKL/KLB pll selection logic to intel_dpll_mgr.c")
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458144418-20046-1-git-send-email-jani.nikula@intel.com