drm/simpledrm: Remove !fb check from atomic_update

The primary plane implements atomic_disable, so atomic_update will
not be called without a framebuffer set. Remove the test for !fb.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220922130944.27138-4-tzimmermann@suse.de