drm/atomic: Add for_each_{connector,crtc,plane}_in_state helper macros

This saves some typing whenever a iteration over all the connector,
crtc or plane states in the atomic state is written, which happens
quite often.

Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>