drm/panel: mantix: Fix panel reset

The mantix panel needs two reset lines (RESX and TP_RSTN) deasserted to
output an image. Only deasserting RESX is not enough and the display
will stay blank. Deassert in prepare() and assert in unprepare() to keep
device held in reset when off.

Fixes: 72967d56 ("drm/panel: Add panel driver for the Mantix MLAF057WE51-X DSI panel")
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/ba71a8ab010d263a8058dd4f711e3bcd95877bf2.1602584953.git.agx@sigxcpu.org