atyfb: hide unused variable

The vdisplay variable is now only accessed inside of an #ifdef, producing
a harmless warning:

drivers/video/fbdev/aty/atyfb_base.c: In function 'aty_var_to_crtc':
drivers/video/fbdev/aty/atyfb_base.c:805:19: error: unused variable 'vdisplay' [-Werror=unused-variable]

This moves the declaration into the ifdef as well.

Fixes: dd7d958a ("video: fbdev: aty: remove useless variable assignments in aty_var_to_crtc()")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: "Gustavo A. R. Silva" <garsilva@embeddedor.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>