drm/i915: do not use 'false' as a NULL pointer

Fixes sparse warning:

drivers/gpu/drm/i915/intel_dpll_mgr.c:1712:24: warning: Using plain
integer as NULL pointer

Fixes: a277ca7d ("drm/i915: Split bxt_ddi_pll_select()")
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Cc: Durgadoss R <durgadoss.r@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473946137-1931-2-git-send-email-jani.nikula@intel.com