drm: Add helper iterator functions for plane fb_damage_clips blob

With fb_damage_clips blob property in drm_plane_state, this patch adds
helper iterator to traverse the damage clips that lie inside plane src.
Iterator will return full plane src as damage in case need full plane
update or damage is not specified.

v2:
- Plane src clipping correction
- Handle no plane update case in iter_next

Signed-off-by: Deepak Rawat <drawat@vmware.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Thomas Hellstrom <thellstrom@vmware.com>
Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>