drm/rockchip: use generic fbdev setup

The Rockchip fbdev code does not add anything compared to
drm_fbdev_generic_setup(); the one custom function for .fb_mmap does the
same thing as gem_prime_mmap which is called by the helper.

Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20211029115014.264084-1-john@metanate.com