drm/fb-helper: Disconnect damage worker from update logic

The fbdev helpers implement a damage worker that forwards fbdev
updates to the DRM driver. The worker's update logic depends on
the generic fbdev emulation. Separate the two via function pointer.

The generic fbdev emulation sets struct drm_fb_helper_funcs.fb_dirty,
a new callback that hides the update logic from the damage worker.
It's not possible to use the generic logic with other fbdev emulation,
because it contains additional code for the shadow buffering that
the generic emulation employs.

DRM drivers with internal fbdev emulation can set fb_dirty to their
own implementation if they require damage handling; although no such
drivers currently exist.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221103151446.2638-16-tzimmermann@suse.de