media: s5p-mfc: Fix potential deadlock on condlock

As &dev->condlock is acquired under irq context along the following
call chain from s5p_mfc_irq(), other acquisition of the same lock
inside process context or softirq context should disable irq avoid double
lock. enc_post_frame_start() seems to be one such function that execute
under process context or softirq context.

<deadlock #1>

enc_post_frame_start()
--> clear_work_bit()
--> spin_loc(&dev->condlock)
<interrupt>
   --> s5p_mfc_irq()
   --> s5p_mfc_handle_frame()
   --> clear_work_bit()
   --> spin_lock(&dev->condlock)

This flaw was found by an experimental static analysis tool I am
developing for irq-related deadlock.

To prevent the potential deadlock, the patch change clear_work_bit()
inside enc_post_frame_start() to clear_work_bit_irqsave().

Signed-off-by: Chengfeng Ye <dg573847474@gmail.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>