drm/ast: Call struct drm_driver.{load, unload} before registering device

Both callbacks are deprecated. Remove them and call functions explicitly.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20191113155857.9507-3-tzimmermann@suse.de