drm/i915: Prevent machine hang from Broxton's vtd w/a and error capture

Since capturing the error state requires fiddling around with the GGTT
to read arbitrary buffers and is itself run under stop_machine(), it
deadlocks the machine (effectively a hard hang) when run in conjunction
with Broxton's VTd workaround to serialize GGTT access.

v2: Store the ERR_PTR in first_error so that the error can be reported
to the user via sysfs.
v3: Mention the quirk in dmesg (using info as per usual)

Fixes: 0ef34ad6222a ("drm/i915: Serialize GTT/Aperture accesses on BXT")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Jon Bloomfield <[email protected]>
Cc: John Harrison <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Daniel Vetter <[email protected]>
Reviewed-by: Joonas Lahtinen <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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