[Intel-gfx] Fixes that failed cherry-pick to 5.10

[Rodrigo Vivi]

Hi Chris and Ville,

Failed to cherry-pick:
233c1ae3c83f ("drm/i915/gt: Wait for CSB entries on Tigerlake")
2c1e63bab430 ("drm/i915: Mark initial fb obj as WT on eLLC machines to avoid rcu lockup during fbdev init")

These patches look important to 5.10. If this is the case,
please consider providing a backported version.

Thanks,
Rodrigo.