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: 304b65cbdc8d ("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

drivers/gpu/drm/i915/intel_dpll_mgr.c

@@ -1196,7 +1196,7 @@ skl_get_dpll(struct intel_crtc *crtc, struct intel_crtc_state *crtc_state,
 		ctrl1 |= DPLL_CTRL1_HDMI_MODE(0);
 
 		if (!skl_ddi_calculate_wrpll(clock * 1000, &wrpll_params))
-			return false;
+			return NULL;
 
 		cfgcr1 = DPLL_CFGCR1_FREQ_ENABLE |
 			 DPLL_CFGCR1_DCO_FRACTION(wrpll_params.dco_fraction) |