i915 intel_frontbuffer_put panic, patch for review/testing

[Ed Maste]

For the last few months I have been experiencing occasional panics
from intel_frontbuffer_put(), while running OBS-studio. An excerpt
from the backtrace:

intel_frontbuffer_put()
intel_user_framebuffer_destroy()
_drm atomic_helper_plane_destroy_state()
drm_atomic_helper_plane_destroy_state()
drm_atomic_state_default_clear()
intel_atomic_state_clear()
__drm_atomic_state_free()
intel_atomic_cleanup_work()
linux_work_fn()

I found a freedesktop.org bug report for an issue that looked
related[1]. Unfortunately the patch attached there does not apply to
Linux 5.4 corresponding to what we have in drm-kmod[2].

I've backported the patch and applied it against the 5.4-lts branch in
the drm-kmod repo, and the issue has not recurred. My patch is in a
pull request at [3], and available as a patch file at [4]. Review
and/or testing is very much appreciated.

[1] https://gitlab.freedesktop.org/drm/intel/-/issues/827
[2] https://lore.kernel.org/stable/15783126692333@kroah.com/
[3] https://github.com/freebsd/drm-kmod/pull/55
[4] https://github.com/freebsd/drm-kmod/commit/b57b441102e9989f5a7d4c9f3c7f29c3790c53f0.patch