Enable the drm panel prepare_prev_first flag so-that the previous
controller should be prepared first before the prepare for the
panel is called.

samsung-s6e3ha2, samsung-s6e63j0x03 and samsung-s6e8aa0 are the
effected samsung-s6e panels for this change.

This makes sure that the previous controller, likely to be a DSI
host controller should be initialized to LP-11 before the panel
is powered up.
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Inki Dae <inki.dae@samsung.com>