drm: Add reference counting to drm_atomic_state

drm: Add reference counting to drm_atomic_state

drm_atomic_state has a complicated single owner model that tracks the
single reference from allocation through to destruction on another
thread - or perhaps on a local error path. We can simplify this tracking
by using reference counting (at a cost of a few more atomics). This is
even more beneficial when the lifetime of the state becomes more
convoluted than being passed to a single worker thread for the commit.

v2: Double check !intel atomic_commit functions for missing gets
v3: Update kerneldocs

Signed-off-by: Chris Wilson <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: [email protected]
Reviewed-by: Eric Engestrom <[email protected]>
Reviewed-by: Sean Paul <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]