fbdev: viafb: use new array-copying-wrapper

viafbdev.c utilizes memdup_user() to copy an array from userspace.

There is a new wrapper, specifically designed for copying arrays. Use
this one instead.

Suggested-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Philipp Stanner <pstanner@redhat.com>
Signed-off-by: Helge Deller <deller@gmx.de>