drivers/gpu/drm/drm_memory.c: fix check for end of loop

"agpmem" is never NULL here.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Cc: Eric Anholt <eric@anholt.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>