drm/msm/sde: check for missed irqs in command encoder

Modify the way that the waiting for previous kickoffs is handled
for the command encoder case. Instead of doing waiting in the
virtual encoder, do the waiting in the command encoder directly.
That way we can check whether we have missed the IRQ. Also
allows waiting for idle before disabling the encoder.

Change-Id: I6cdb9c961a0ffd8e9ea2aaf1ad9686757edd7142
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>