[Why]
The behavior of drm_atomic_helper_cleanup_planes differs depending on
whether the commit was asynchronous or not. When it's called from
amdgpu_dm_atomic_commit_tail during a typical atomic commit the
plane state has been swapped so it calls cleanup_fb on the old plane
state.

However, in the asynchronous commit codepath the call to
drm_atomic_helper_commit also calls dm_plane_helper_cleanup_fb after
atomic_async_update has been called. Since the plane state is updated
in place and has not been swapped the cleanup_fb call affects the new
plane state.

This results in a use after free for the given sequence:

- Fast update, fb1 pin/ref, fb1 unpin/unref
- Fast update, fb2 pin/ref, fb2 unpin/unref
- Slow update, fb1 pin/ref, fb2 unpin/unref
- Fast update, fb2 pin/ref -> use after free. bug

[How]
Disallow framebuffer changes in the fast path. Since this includes
a NULL framebuffer, this means that only framebuffers that have
been previously pin+ref at least once will be used, preventing a
use after free.

This has a significant throughput reduction for cursor updates where
the framebuffer changes. For most desktop usage this isn't a problem,
but it does introduce performance regressions for two specific IGT
tests:

- cursor-vs-flip-toggle
- cursor-vs-flip-varying-size

Fixes: 2cc751931afc ("drm/amd/display: Add fast path for cursor plane updates")
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Reviewed-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>