drm/panel: jh057n00900: Use drm_panel_{unprepare, disable} consistently

We were already using the generic functions in our debugfs code, do the
same in jh057n_shutdown. This was suggested by Sam Ravnborg.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/a37dd5083462064f437ff62fd84e6576d8a7c8dc.1564146727.git.agx@sigxcpu.org