fbdev: fix integer as NULL pointer warning

drivers/video/aty/atyfb_base.c:3359:26: warning: Using plain integer as NULL pointer
drivers/video/aty/radeon_base.c:2280:32: warning: Using plain integer as NULL pointer
drivers/video/matrox/matroxfb_base.h:203:25: warning: Using plain integer as NULL pointer
drivers/video/matrox/matroxfb_base.h:203:25: warning: Using plain integer as NULL pointer
drivers/video/sis/sis_main.c:5790:44: warning: Using plain integer as NULL pointer

Signed-off-by: Harvey Harrison <harvey.harrison@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/drivers/video/aty/atyfb_base.c b/drivers/video/aty/atyfb_base.c
index e4bcf53..bd4ac0b 100644
--- a/drivers/video/aty/atyfb_base.c
+++ b/drivers/video/aty/atyfb_base.c
@@ -3356,7 +3356,7 @@
 
 	info->fix.mmio_start = raddr;
 	par->ati_regbase = ioremap(info->fix.mmio_start, 0x1000);
-	if (par->ati_regbase == 0)
+	if (par->ati_regbase == NULL)
 		return -ENOMEM;
 
 	info->fix.mmio_start += par->aux_start ? 0x400 : 0xc00;