video: fbdev: dnfb: make fb_var_screeninfo static and const

Make this structure static as it is not referenced in any other
file. Make it const as it is used only during a copy operation.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Cc: Julia Lawall <julia.lawall@lip6.fr>
[b.zolnierkie: split from combined patch]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>