drm/panel: panel-simple: Add missing pm_runtime_disable() calls

In commit 3235b0f2 ("drm/panel: panel-simple: Use runtime pm to
avoid excessive unprepare / prepare") we started using pm_runtime, but
my patch neglected to add the proper pm_runtime_disable(). Doh! Add
them now.

Fixes: 3235b0f2 ("drm/panel: panel-simple: Use runtime pm to avoid excessive unprepare / prepare")
Reported-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20210423095743.v5.1.I9e6af2529d6c61e5daf86a15a1211121c5223b9a@changeid