drm/panel: simple: Add missing panel_simple_unprepare() calls

During panel removal or system shutdown panel_simple_disable() is called
which disables the panel backlight but the panel is still powered due to
missing calls to panel_simple_unprepare().

Fixes: d02fd93e ("drm/panel: simple - Disable panel on shutdown")
Cc: stable@vger.kernel.org # v3.16+
Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170807115545.27747-1-net147@gmail.com