drm/fbdev-ttm: Support struct drm_driver.fbdev_probe

Rework fbdev probing to support fbdev_probe in struct drm_driver
and reimplement the old fb_probe callback on top of it. Provide an
initializer macro for struct drm_driver that sets the callback
according to the kernel configuration.

This change allows the common fbdev client to run on top of TTM-
based DRM drivers.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240924071734.98201-65-tzimmermann@suse.de