video: fbdev: annotate fb_fix_screeninfo with const and __initconst

Make these const as they are only used during a copy operation.
Some structures are used as a copy operation inside __init functions, so
make them const and replace __initdata with __initconst to avoid section
conflict error.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Cc: Julia Lawall <julia.lawall@lip6.fr>
Cc: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>