drm/msm/sde: signal release fence after flush accepted

Avoid signaling the release fence for video mode until after
the flush bits have been accepted by the hardware. This
ensures that the new buffers and configuration are in use
before old buffers are released back to the system.

Change-Id: I0a112a16e14a0f002fcb385da71084d209344c26
Signed-off-by: Clarence Ip <cip@codeaurora.org>