drm/i915: document caching related bits

Try to document the object caching related bits, like cache_coherent and
cache_dirty.

v2(Ville):
 - As pointed out by Ville, fix the completely incorrect assumptions
   about the "partial" coherency on shared LLC platforms.
v3(Daniel):
 - Fix nonsense about "dirtying" the cache with reads.
v4(Daniel):
 - Various improvements, including adding some more details for WT.

Suggested-by: Daniel Vetter <[email protected]>
Signed-off-by: Matthew Auld <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Reviewed-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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