drm/udl: Use NULL instead of 0 for pointers

Fixes the following sparse warnings:
drivers/gpu/drm/udl/udl_transfer.c:129:50:
drivers/gpu/drm/udl/udl_transfer.c:130:50:
drivers/gpu/drm/udl/udl_transfer.c:131:45:
drivers/gpu/drm/udl/udl_transfer.c:132:61:
warning: Using plain integer as NULL pointer
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/udl/udl_transfer.c

@@ -126,10 +126,10 @@ static void udl_compress_hline16(
 
 	while ((pixel_end > pixel) &&
 	       (cmd_buffer_end - MIN_RLX_CMD_BYTES > cmd)) {
-		uint8_t *raw_pixels_count_byte = 0;
-		uint8_t *cmd_pixels_count_byte = 0;
-		const u8 *raw_pixel_start = 0;
-		const u8 *cmd_pixel_start, *cmd_pixel_end = 0;
+		uint8_t *raw_pixels_count_byte = NULL;
+		uint8_t *cmd_pixels_count_byte = NULL;
+		const u8 *raw_pixel_start = NULL;
+		const u8 *cmd_pixel_start, *cmd_pixel_end = NULL;
 
 		prefetchw((void *) cmd); /* pull in one cache line at least */