agp: convert drivers/char/agp/frontend.c to use unlocked_ioctl

As of now, agp_compat_ioctl already runs without the BKL.  Mutual exclusion
is enforced by agp_fe.agp_mutex in agp_ioctl() and agp_compat_ioctl().
Apply the same locking rationale to the two functions allowing BKL cleanup.

Signed-off-by: Mathieu Segaud <mathieu.segaud@regala.cx>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@linux.ie>