drm/i915/selftest: Clear the output buffers before GPU writes

When testing whether we can get the GPU to leak information about
non-privileged state, we first need to ensure that the output buffer is
set to a known value as the HW may opt to skip the write into memory for
a non-privileged read of a sensitive register. We chose POISON_INUSE (0x5a)
so that is both non-zero and distinct from the poison values used during
the test.

v2:
  Use i915_gem_object_pin_map_unlocked

Reported-by: CQ Tang <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Cc: CQ Tang <[email protected]>
cc: Joonas Lahtinen <[email protected]>
Signed-off-by: Ramalingam C <[email protected]>
Reviewed-by: Thomas Hellstrom <[email protected]>
Signed-off-by: Karolina Drobnik <[email protected]>
Reviewed-by: Andi Shyti <[email protected]>
Signed-off-by: Andi Shyti <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/5cebab02d182c171cf40cb5b73d6c3eeb7619360.1663081418.git.karolina.drobnik@intel.com

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