fbdev: omapfb: avoid -Wempty-body warning

Building with 'make W=1' shows a few harmless warnings:

drivers/video/fbdev/omap2/omapfb/omapfb-main.c: In function 'omapfb_calc_addr':
drivers/video/fbdev/omap2/omapfb/omapfb-main.c:823:56: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
  823 |                     var->xoffset, var->yoffset, offset);
      |                                                        ^
drivers/video/fbdev/omap2/omapfb/omapfb-ioctl.c: In function 'omapfb_ioctl':
drivers/video/fbdev/omap2/omapfb/omapfb-ioctl.c:911:45: error: suggest braces around empty body in an 'if' statement [-Werror=empty-body]
  911 |                 DBG("ioctl failed: %d\n", r);

Avoid these by using no_printk(), which adds format string checking as
well.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210322105307.1291840-1-arnd@kernel.org

drivers/video/fbdev/omap2/omapfb/omapfb.h

@@ -29,7 +29,7 @@ extern bool omapfb_debug;
 			printk(KERN_DEBUG "OMAPFB: " format, ## __VA_ARGS__); \
 	} while (0)
 #else
-#define DBG(format, ...)
+#define DBG(format, ...) no_printk(format, ## __VA_ARGS__)
 #endif
 
 #define FB2OFB(fb_info) ((struct omapfb_info *)(fb_info->par))