drm/mcde: Don't use drm_device->dev_private

Upcasting using a container_of macro is more typesafe, faster and
easier for the compiler to optimize.

v2: Move misplaced removal of double-assignment to this patch (Sam)

Reviewed-by: Linus Walleij <linus.walleij@linaro.org> (v1)
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20200415074034.175360-30-daniel.vetter@ffwll.ch