rockchip/vcodec: fix crash on decoder buffer empty

rockchip/vcodec: fix crash on decoder buffer empty

On vpu2 register separate interrupt bit and enable bit to different
register. When decoder found a buffer empty error which means the input
stream is not enough for one complete frame decoder will not stop
reading input stream buffer until it reach the end of buffer. This will
cause mmu fault on the buffer end.

In order to avoid this case decoder need to clear the enable bit in the
enable register to stop decoder from reading.

Change-Id: I6133aa4611fab03f6545b4775e8ee2320552445f
Signed-off-by: Herman Chen <herman.chen@rock-chips.com>