drm/i915: Flush WC GGTT only on required platforms

gen8_ggtt_invalidate() is only needed for limited set of platforms
where GGTT is mapped as WC. This was added as way to fix WC based GGTT in
commit 0f9b91c754b7 ("drm/i915: flush system agent TLBs on SNB") and
there are no reference in HW docs that forces us to use this on non-WC
backed GGTT.

This can also cause unwanted side-effects on XE_HP platforms where
GFX_FLSH_CNTL_GEN6 is not valid anymore.

v2: Add a func to detect wc ggtt detection (Ville)
v3: Improve commit log and add reference commit (Daniel)

Fixes: d2eae8e98d59 ("drm/i915/dg2: Drop force_probe requirement")
Cc: Rodrigo Vivi <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Jani Nikula <[email protected]>
Cc: Jonathan Cavitt <[email protected]>
Cc: John Harrison <[email protected]>
Cc: Andi Shyti <[email protected]>
Cc: Ville Syrjälä <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: <[email protected]> # v6.2+
Suggested-by: Matt Roper <[email protected]>
Signed-off-by: Nirmoy Das <[email protected]>
Reviewed-by: Matt Roper <[email protected]>
Reviewed-by: Andi Shyti <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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