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

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

Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Cc: Russell King <linux@armlinux.org.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200904143941.110665-3-daniel.vetter@ffwll.ch