drm/i915: Use the same vblank worker for atomic unpin

In case of legacy cursor update, the cursor VMA needs to be unpinned
only after vblank. This exceeds the lifetime of the whole atomic commit.

Any trick I attempted to keep the atomic commit alive didn't work, as
drm_atomic_helper_setup_commit() force throttles on any old commit that
wasn't cleaned up.

The only option remaining is to remove the plane from the atomic commit,
and use the same path as the legacy cursor update to clean the state
after vblank.

Changes since previous version:
- Call the memset for plane state immediately when scheduling vblank,
  this prevents a use-after-free in cursor cleanup.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Uma Shankar <uma.shankar@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240522053341.137592-4-maarten.lankhorst@linux.intel.com