If the panel is wrapped in a panel_bridge it gets prepar()ed before the
upstream DSI bridge which can cause hangs (e.g. with imx-nwl since clocks
are not enabled yet). To avoid this move the panel's first DSI access to
enable() so the upstream bridge can prepare the DSI host controller in
it's pre_enable().

This is also in line with other panel drivers.
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/12c3495b234952aafe11980a9e06cfd246134660.1564146727.git.agx@sigxcpu.org