drm/i915/selftests: Skip poisoning SET_PREDICATE_RESULT on dg2

When predication is enabled all commands baring a few (such as MI_BB_END)
are nop'ed. If we accidentally enable predication while poisoning the
context, not only is the rest of the poisoning skipped (thus disabling
the test), but the closing instructions of the poison request are
nop'ed. Not only do we then not signal the waiting context, but we even
prevent re-enabling arbitration and the GPU will not perform a context
switch at the end of the request.

Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Suggested-by: CQ Tang <cq.tang@intel.com>
Signed-off-by: Chris Wilson <chris.p.wilson@intel.com>
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220425152317.4275-3-ramalingam.c@intel.com