drm/panel: simple: tv123wam: Add unprepare delay

The panel datasheet specifies a 500ms delay after power-down before
re-enabling.

Changes in v2:
- None
Changes in v3:
- Added to the set

Cc: Sandeep Panda <spanda@codeaurora.org>
Reviewed-by: Sandeep Panda <spanda@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180813213058.184821-2-sean@poorly.run