drm/gma500: Replace references to dev_private with helper function

Replace most references to struct drm_device.dev_private with the new
helper function to_drm_psb_private(). The only references left are in
assignments and the helper itself.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210920141051.30988-2-tzimmermann@suse.de