video: fbdev: vt8623fb: constify vt8623_timing_regs

vt8623_timing_regs is only passed as the first argument to
svga_check_timings and as the second argument to svga_set_timings,
both of which are const.  So make vt8623_timing_regs const also.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Cc: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>