Fixes the following sparse warnings:

 drivers/char/drm/via_dmablit.c:111:35: warning: Using plain integer as NULL pointer
 drivers/char/drm/via_dmablit.c:584:23: warning: Using plain integer as NULL pointer
Signed-off-by: Luiz Capitulino <lcapitulino@mandriva.com.br>
Signed-off-by: Dave Airlie <airlied@linux.ie>