drm/fb-helper: Remove test for fb_dirty callback from deferred-I/O helper

The helper for processing deferred I/O on pages has no dependency on
the fb_dirty damge-handling callback; so remove the test. In practice,
deferred I/O is only used with damage handling and the damage worker
already guarantees the presence of the fb_dirty callback.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20221115115819.23088-4-tzimmermann@suse.de