drm/i915/gem: Fix NULL deref in I915_CONTEXT_PARAM_SSEU

drm/i915/gem: Fix NULL deref in I915_CONTEXT_PARAM_SSEU

Setting context engine slot N into I915_ENGINE_CLASS_INVALID /
I915_ENGINE_CLASS_INVALID_NONE and attempting to apply
I915_CONTEXT_PARAM_SSEU to the same slot N will deref NULL.
Fix that.

Discovered using AI-assisted static analysis confirmed by
Intel Product Security.

Reported-by: Martin Hodo <martin.hodo@intel.com>
Fixes: d4433c7600f7 ("drm/i915/gem: Use the proto-context to handle create parameters (v5)")
Cc: Faith Ekstrand <faith.ekstrand@collabora.com>
Cc: Simona Vetter <simona.vetter@ffwll.ch>
Cc: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: <stable@vger.kernel.org> # v5.15+
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patch.msgid.link/20260701075555.52142-1-joonas.lahtinen@linux.intel.com
(cherry picked from commit 36eda5b5c2d40da41cc0a5403c26986237cf9e87)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>