drm/i915: Mark ininitial fb obj as WT on eLLC machines to avoid rcu lockup during fbdev init

Currently we leave the cache_level of the initial fb obj
set to NONE. This means on eLLC machines the first pin_to_display()
will try to switch it to WT which requires a vma unbind+bind.
If that happens during the fbdev initialization rcu does not
seem operational which causes the unbind to get stuck. To
most appearances this looks like a dead machine on boot.

Avoid the unbind by already marking the object cache_level
as WT when creating it. We still do an excplicit ggtt pin
which will rewrite the PTEs anyway, so they will match whatever
cache level we set.

Cc: <[email protected]> # v5.7+
Suggested-by: Chris Wilson <[email protected]>
Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/2381
Signed-off-by: Ville Syrjälä <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
(cherry picked from commit d46b60a2e8d246f1f0faa38e52f4f5a73858c338)
Signed-off-by: Rodrigo Vivi <[email protected]>

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