drm/cirrus: Use drm_fb_blit() to update scanout buffer

Cirrus' blit helper reimplements code from the shared blit helper
drm_fb_blit(). Use the helper instead.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215161517.5113-4-tzimmermann@suse.de