video: fbdev: make fb_var_screeninfo const

Make these const as they are not modified anywhere.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Cc: Guan Xuetao <gxt@mprc.pku.edu.cn>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>