video: xilinxfb: constify fb_fix_screeninfo and fb_var_screeninfo structures

These structures are only used to copy into other structures,
so declare them as const.

This issue was detected using Coccinelle and the following semantic patch:

@r disable optional_qualifier@
identifier i;
position p;
@@
static struct fb_fix_screeninfo i@p = { ... };

@ok@
identifier r.i;
expression e;
position p;
@@
e = i@p

@bad@
position p != {r.p,ok.p};
identifier r.i;
struct fb_fix_screeninfo e;
@@
e@i@p

@depends on !bad disable optional_qualifier@
identifier r.i;
@@
static
+const
 struct fb_fix_screeninfo i = { ... };

The semantic patch for fb_var_screeninfo is analogous.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Cc: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>