fbdev: matrox: hide unused 'hotplug' variable

The variable has become unused in modular configurations
which triggers a harmless warning:

drivers/video/fbdev/matrox/matroxfb_base.c:1583:12: error: 'hotplug' defined but not used [-Werror=unused-variable]

This moves it into an #ifdef section of the file, matching
all its references.

Fixes: 376b3ff5 ("fbdev: Nuke FBINFO_MODULE")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>