drm/gma500: Call psb_driver_{load, unload}() before registering device

The load/unload callbacks in struct drm_driver are deprecated. Remove
them and call functions explicitly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191204184617.22201-1-tzimmermann@suse.de