drm/gma500: Embed struct drm_device in struct drm_psb_private

Embed struct drm_device in struct drm_psb_private. Replace the use
of dev_private by an upcast operation. Switch to managed release of
struct drm_psb_private.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210920141051.30988-4-tzimmermann@suse.de