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