drm: Consolidate plane arrays in drm_atomic_state

drm: Consolidate plane arrays in drm_atomic_state

It's kinda pointless to have 2 separate mallocs for these. And when we
add more per-plane state in the future it's even more pointless.

Right now there's no such thing planned, but both Gustavo's per-crtc
fence patches, and some nonblocking commit helpers I'm playing around
with will add more per-crtc stuff. It makes sense to also consolidate
planes, just for consistency.

In the future we can use this to store a pointer to the preceeding
state, making an atomic update entirely free-standing. This will be
needed to be able to queue them up with a depth > 1.

Cc: Gustavo Padovan <gustavo@padovan.org>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1464818821-5736-11-git-send-email-daniel.vetter@ffwll.ch