fbdev: fbmon: fix -Wextra build warnings

When 'DEBUG' is not defined, modify the DPRINTK() macro to use the
no_printk() macro instead of using <empty>.
This fixes a build warning when -Wextra is used and provides
printk format checking:

../drivers/video/fbdev/core/fbmon.c:812:47: warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]
../drivers/video/fbdev/core/fbmon.c:842:24: warning: suggest braces around empty body in an ‘else’ statement [-Wempty-body]
../drivers/video/fbdev/core/fbmon.c:847:24: warning: suggest braces around empty body in an ‘else’ statement [-Wempty-body]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Antonino Daplas <adaplas@gmail.com>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200315041002.24473-2-rdunlap@infradead.org