drm/msm/dsi: check video mode engine status before waiting

Make sure the video mode engine is on before waiting
for the video done interrupt.

Changes in v4:
- Move setting enabled to false earlier

Changes in v3:
- Move the return value check to another
  patch

Changes in v2:
- Replace pr_err with dev_err
- Changed error message

Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>