drm/panel: novatek,nt39016: Remove 'dev' field in priv struct

There is already a 'struct device' pointer in the drm_panel structure,
that we can access easily from our priv structure, so there's no need
for a separate 'dev' field there.

Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20200820121256.32037-3-paul@crapouillou.net