drivers/video/mmp/core.c: fix use-after-free bug

Found with coccinelle.

Signed-off-by: Andrei Epure <epure.andrei@gmail.com>
Cc: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>