drm/damage-helper: Add drm_atomic_helper_damage_merged()

Useful for drivers that only care about the combined damage.

v2: Remove unnecessary clearing of damage clips

Cc: Deepak Rawat <drawat@vmware.com>
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190115043643.2364-3-noralf@tronnes.org