drm: Remove struct drm_driver.gem_print_info

The .gem_print_info callback in struct drm_driver is obsolete and has
no users left. Remove it.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Suggested-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605073247.4057-44-tzimmermann@suse.de