drm/atomic: document how to handle driver private objects

DK put some nice docs into the commit introducing driver private
state, but in the git history alone it'll be lost.

Also, since Ville remove the void* usage it's a good opportunity to
give the driver private stuff some tlc on the doc front.

Finally try to explain why the "let's just subclass drm_atomic_state"
approach wasn't the greatest, and annotate all those functions as
deprecated in favour of more standardized driver private states. Also
note where we could/should extend driver private states going forward
(atm neither locking nor synchronization is handled in core/helpers,
which isn't really all that great).

v2: Spelling and phrasing improvements (Alex, DK).

Cc: Harry Wentland <[email protected]>
Cc: Dhinakaran Pandiyan <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: Laurent Pinchart <[email protected]>
Cc: Rob Clark <[email protected]>
Cc: Alex Deucher <[email protected]>
Cc: Ben Skeggs <[email protected]>
Reviewed-by: Dhinakaran Pandiyan <[email protected]>
Reviewed-by: Alex Deucher <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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