drm/fb-helper: Restore damage area upon errors

If the damage handling fails, restore the damage area. The next invocation
of the damage worker will then perform the update.

v3:
	* Use drm_WARN_ONCE() with an error message to print warning
v2:
	* print a single warning if dirty callback fails (Daniel, Sebastian)
	* update comment

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201120102545.4047-9-tzimmermann@suse.de