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 <daniel.vetter@ffwll.ch>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210723105045.400841-1-matthew.auld@intel.com