drm/fb-helper: Return early in damage worker

Returning early in the damage worker if no update is required. Makes the
code more readable. No functional changes are being made.

v3:
	* s/dirty/damage in commit message (Sam)

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Maxime Ripard <mripard@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201120102545.4047-6-tzimmermann@suse.de