drm: don't block fb changes for async plane updates

In the case of a normal sync update, the preparation of framebuffers (be
it calling drm_atomic_helper_prepare_planes() or doing setups with
drm_framebuffer_get()) are performed in the new_state and the respective
cleanups are performed in the old_state.

In the case of async updates, the preparation is also done in the
new_state but the cleanups are done in the new_state (because updates
are performed in place, i.e. in the current state).

The current code blocks async udpates when the fb is changed, turning
async updates into sync updates, slowing down cursor updates and
introducing regressions in igt tests with errors of type:

"CRITICAL: completed 97 cursor updated in a period of 30 flips, we
expect to complete approximately 15360 updates, with the threshold set
at 7680"

Fb changes in async updates were prevented to avoid the following scenario:

- Async update, oldfb = NULL, newfb = fb1, prepare fb1, cleanup fb1
- Async update, oldfb = fb1, newfb = fb2, prepare fb2, cleanup fb2
- Non-async commit, oldfb = fb2, newfb = fb1, prepare fb1, cleanup fb2 (wrong)
Where we have a single call to prepare fb2 but double cleanup call to fb2.

To solve the above problems, instead of blocking async fb changes, we
place the old framebuffer in the new_state object, so when the code
performs cleanups in the new_state it will cleanup the old_fb and we
will have the following scenario instead:

- Async update, oldfb = NULL, newfb = fb1, prepare fb1, no cleanup
- Async update, oldfb = fb1, newfb = fb2, prepare fb2, cleanup fb1
- Non-async commit, oldfb = fb2, newfb = fb1, prepare fb1, cleanup fb2

Where calls to prepare/cleanup are balanced.

Cc: <stable@vger.kernel.org> # v4.14+
Fixes: 25dc194b

 ("drm: Block fb changes for async plane updates")
Suggested-by: Boris Brezillon <boris.brezillon@collabora.com>
Signed-off-by: Helen Koike <helen.koike@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190603165610.24614-6-helen.koike@collabora.com