drm/i915: use NULL for NULL pointers

Fix sparse warning:

drivers/gpu/drm/i915/i915_cmd_parser.c:987:72: warning: Using plain
integer as NULL pointer

Fixes: 52a42cec ("drm/i915/cmdparser: Accelerate copies from WC memory")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1473946137-1931-4-git-send-email-jani.nikula@intel.com

drivers/gpu/drm/i915/i915_cmd_parser.c

@@ -984,7 +984,7 @@ static u32 *copy_batch(struct drm_i915_gem_object *dst_obj,
 
 	src = ERR_PTR(-ENODEV);
 	if (src_needs_clflush &&
-	    i915_memcpy_from_wc((void *)(uintptr_t)batch_start_offset, 0, 0)) {
+	    i915_memcpy_from_wc((void *)(uintptr_t)batch_start_offset, NULL, 0)) {
 		src = i915_gem_object_pin_map(src_obj, I915_MAP_WC);
 		if (!IS_ERR(src)) {
 			i915_memcpy_from_wc(dst,