drm/panel-notatek-nt35510: Fix MTP read init

In order to successfully read ID of the MTP panel the
panel MTP control page must be unlocked. Previously
this wasn't encountered because in the setup with this
panel the power wasn't ever really dropped. When power
gets dropped from the panel, MTP needs to be unlocked.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Cc: newbytee@protonmail.com
Cc: Stephan Gerhold <stephan@gerhold.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200808224322.1507713-1-linus.walleij@linaro.org