video: fbdev: xilinxfb: constify copied structure

Make const xilinx_fb_default_pdata structure that is only copied into
other structure.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: Soren Brinkmann <soren.brinkmann@xilinx.com>,
[b.zolnierkie: split from a bigger patch]
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>