drm/i915: Fix global state use-after-frees with a refcount

While the current locking/serialization of the global state
suffices for protecting the obj->state access and the actual
hardware reprogramming, we do have a problem with accessing
the old/new states during nonblocking commits.

The state computation and swap will be protected by the crtc
locks, but the commit_tails can finish out of order, thus also
causing the atomic states to be cleaned up out of order. This
would mean the commit that started first but finished last has
had its new state freed as the no-longer-needed old state by the
other commit.

To fix this let's just refcount the states. obj->state amounts
to one reference, and the intel_atomic_state holds extra references
to both its new and old global obj states.

Fixes: 0ef1905ecf2e ("drm/i915: Introduce better global state handling")
Signed-off-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Stanislav Lisovskiy <[email protected]>
(cherry picked from commit f8c86ffa2800adc80adc679c84c45e0c6b027374)
Signed-off-by: Joonas Lahtinen <[email protected]>

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