drm: Fixes sparse warnings in via_dmablit.c

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>

drivers/char/drm/via_dmablit.c

@@ -108,7 +108,7 @@ via_map_blit_for_device(struct pci_dev *pdev,
 	int num_desc = 0;
 	int cur_line;
 	dma_addr_t next = 0 | VIA_DMA_DPR_EC;
-	drm_via_descriptor_t *desc_ptr = 0;
+	drm_via_descriptor_t *desc_ptr = NULL;
 
 	if (mode == 1) 
 		desc_ptr = vsg->desc_pages[cur_descriptor_page];
@@ -581,7 +581,7 @@ via_build_sg_info(drm_device_t *dev, drm_via_sg_info_t *vsg, drm_via_dmablit_t *
 	int ret = 0;
 	
 	vsg->direction = (draw) ? DMA_TO_DEVICE : DMA_FROM_DEVICE;
-	vsg->bounce_buffer = 0;
+	vsg->bounce_buffer = NULL;
 
 	vsg->state = dr_via_sg_init;