drm/i915: Fix compute pre-emption w/a to apply to compute engines

An earlier patch added support for compute engines. However, it missed
enabling the anti-pre-emption w/a for the new engine class. So move
the 'compute capable' flag earlier and use it for the pre-emption w/a
test.

Fixes: c674c5b9 ("drm/i915/xehp: CCS should use RCS setup functions")
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Aravind Iddamsetty <aravind.iddamsetty@intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Lucas De Marchi <lucas.demarchi@intel.com>
Cc: John Harrison <John.C.Harrison@Intel.com>
Cc: Jason Ekstrand <jason@jlekstrand.net>
Cc: "Michał Winiarski" <michal.winiarski@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tejas Upadhyay <tejaskumarx.surendrakumar.upadhyay@intel.com>
Cc: Umesh Nerlige Ramappa <umesh.nerlige.ramappa@intel.com>
Cc: "Thomas Hellström" <thomas.hellstrom@linux.intel.com>
Cc: Stuart Summers <stuart.summers@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Ramalingam C <ramalingam.c@intel.com>
Cc: Akeem G Abodunrin <akeem.g.abodunrin@intel.com>
Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221006213813.1563435-3-John.C.Harrison@Intel.com