media: exynos: mfc: don't drop wakeup source in multiple context case

Wakeup sources aren't ref counted, but we we always dropping ours in
release. If there were multiple contexts open, it would allow suspend
even though the MFC was still in use. Although, by itself, it's not a
problem, it did not have the intended effect.

Bug: 7261738
Change-Id: Id42006b2cff0e84711047a149a8654ed950f012a
Signed-off-by: Dima Zavin <dima@android.com>