viafb: fix accel_flags check_var bug

In check_var we should check and modify the var given and not the
one which is currently active. So this code was obviously wrong.
Probably this was doing no harm because all acceleration functions
also check whether acceleration is possible. (otherwise I would
expect this to lead to a null pointer dereference)
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Cc: Joseph Chan <JosephChan@via.com.tw>